This has been covered well on other sites.  If you look on Richard Bevan's Saab Audio site, there's plenty of info there.  It's the most simple thing you can do to upgrade your sound, and it's highly recommended. 

Another excellent site is Munki's site where you'll find good info on a variety of 900NG issues.

Well, there are a few sites out there that focus on Saab 900 NG audio upgrades, but none that I've found that include information on convertibles.  My Saab is a '96 900S (non turbo) model, in the USA. 

First, we'll look at the rear speakers, which presented me with some unique problems.  Removing the side panel is simple.  (It's required, DO NOT just pull on the grill, you will break it and you won't be able to change the speakers that way anyway). 

To remove the panel:

• Like the door speakers, there are a few torx screws.  Remove them.  There is one that is hidden by a small cover in the door opening. 
• There are also some black plastic clips that simply require a flat head screwdriver and a half a turn.  They pop right out.  Some you can get at right away, but a few you have to remove the rear seat to access. 
• The rear seat is easy to take off.  Just pull up and then back.  It pulls right out. 

Once you get all the screws and plastic clips removed, here's what you'll see:

Notice the speaker is mounted in a mounting plate.  This plate needs to removed to change the speaker.  You just remove the torx screws and it comes off. 

Here's the plate removed, with my new speaker attached.  Note I had to do some surgery to get the 6.5" speaker (JL Audio) to fit properly.  I had to either bend, or cut off a portion of the metal lip of the speaker.  Otherwise it would not fit.  The original speaker is NOT exactly round, so it fits fine.  I think most aftermarket speakers are round, so you'll have to do this to yours too, I'd imagine, unless you went with a 5.25" speaker and an adapter.  It's easy enough to do, though.  The first one I used tin snips which took a long time, but for the second one I used vice grips to bend it, that was easier, quicker, and worked just as well.

I was a little worried about the way the plate was mounted, because it didn't look like there was a good seal between the panel and the speaker.  After looking closely at the design, the foam surround should provide enough of a seal to make it work.  I may use some leftover dynamat later to try and achieve a better seal, but I'll decide after I listen to my system for a while. 

Note the speakers I put it are just 6.5" woofers, no tweeters.  You'll see more about why I did this below, where I describe my system. 

That's pretty much all there is to it... connect the speaker (make sure your polarity is correct... it's kind of a pain to do because the stock speaker doesn't have polarity marked.  I used a volt ohm meter, but if you mark the wires "left and right" for each side and connect them similarly, you will be fine. 

This is where the pain sets in, as others have reported.  I was pretty nervous about trying this, but it wasn't quite as bad as I thought it would be... but it WAS difficult.  I ended up leaving my door panel off for the better part of a week before I had a game plan that I thought would work well. 

Remove the door panel.  Richard Bevan's site describes this well, where the torx screws are, etc.  One note... to remove the panel, after you get the torx screws out, carefully push the panel over the lock, then pull straight out... don't use too much force, you don't need to. I didn't break any plastic clips, thank goodness. 

The first thing to do is pull out the foam piece covering the speaker hole. 

Then make sure you remove the foam in the lining... it's already perforated. 

Oh, and don't forget to remove the cutout piece from the door covering.  I forgot to do this and had to remove one door panel again.  I guess I'm getting pretty good at it now...

Next, you have to figure out if you have wiring in place between the door cutout and the amp location.  I didn't, of course, but if you look at the picture, it sure looks like the amp wiring is in place... for whatever reason, it's not connected, at least not that I could find.  I even tried to find the connection in the multipin cable in the door, thinking maybe I just needed to pull a cable from there, but no go. 

So here's what I did... it was difficult to do, but I pulled open the grommet (there are two, the outside one shown, but there's also one inside).  Of course, the inside grommet doesn't line up exactly with the outside one, so getting the wires through was a challenge... but it worked. 

Then, I drilled a small hole just large enough for the speaker wire and a small rubber grommet.  I put it as close to the multipin cable as possible to allow it to be hidden well, on the angled section behind.  This makes it a bit harder to run the wiring on the other side, but it makes it easier to hide the speaker cable. 

You have no idea how good it felt to see this!!

Here's the final product... not perfectly pro, but good enough.  If you look closely, you'll see the opening and the speaker cable just above where the multipin cable connects.  The plastic cable tie is just used to keep the speaker cable in place. 

It was a pain, no doubt about it.  I have no idea how long it took me to actually do, because I spent far more time trying to decide how to actually do it!  There may be a better way, but it works for me.  It was pretty nerve racking drilling into the door panel, but I didn't see another way. 

I should have taken a few more pictures, but this will help anyway.  I had some Boston Acoustic 5.25" speakers that looked like they would fit perfectly in the openings.. it took me a long time to figure out the best way to mount them.  Simple!

There's very little room in the opening to screw into.. i.e., the diameter of the hole is just small enough to allow mounting the speakers, but there's very little metal between the hole and the screws you'll use to mount the speaker.  The way I did it was to drill a bit further out, so the screws would mount outwards at a slight angle.  It worked fine. 

I would say some dynamat is a requirement here, because there is a wider opening at the bottom (probably for the stock Saab mounting plate).  I didn't look into just buying the plate, maybe that would work too.  Notice the dynamat in the picture surrounding the speaker. 

I had a Nakamichi head unit from another car that I bought for its excellent sound quality, AND its stock look.  I thought it would look pretty good in my Saab, and sound a lot better then the head unit.  Plus it has an aux in, and an iPod purchase is eventually in the cards. 

I bought the wiring harness from genuinesaab.com, and hooked up the wires in the comfort of my own home.  It worked perfectly right off the bat.  I also bought the installation kit, which includes the pocket to make the installation look good..

Here's how it looks...

The only thing I don't like is the color of the Nakamichi is black, and the Saab is more of a dark gray.  However, it looks closer to stock then pretty much anything else I've seen in a Saab.  The Nak head unit's lighting seems to match the color of the Saab interior pretty well (amber lighting), but I confess I haven't seen it in the dark yet because I just installed it today!

Here's a shot of my Saab... I do need to wash it now!

OK, here's a description of my system:

Head unit:  Nakamichi CD-40z

Front speakers:  Boston Acoustics tweeters only (part of component set with 5.25 mid bass in doors)

Door speakers:  Boston Acoustics 5.25" mid/bass drivers (component)

Rear speakers:  JL Audio 6.5" woofers (component)

The front and door speakers are running off the "front" channel of the Nak head.  The JL Audio speakers are running off the rear channel.

The front and door speakers have a Boston Acoustics crossover installed running at about 2200 hz.  I also installed a "bass blocker" (capacitor) rolling off audio below 150hz.  The JL Audio 6.5s in the back currently have no cross over, but I will be installing inductors in-line to remove everything above 200hz.  This will improve the imaging.  Parts are on order.

The sound?  Great!  But I need to make some modifications.  I think I'm eventually going to need to install a separate amp for the rears, as I suspected.  The front and door speakers will run off front and rear power of the Nak, and then I'll add a small maybe 75 watt RMS amp for the rears.  I'm hoping there's room in the kick panel location (where I currently have the crossovers) for a relatively small amp... I think there is, but we'll see.  It's definitely work in process... but so far, so good!

One thing to note about convertibles... if you have the top down a lot, you need a lot more power!  I have my top down a lot, since I'm lucky enough to be living in California.  If you own a convertible, my recommendation is to include more power in your design of your audio system.  

Finally, thanks to:

Saab Central, a great group of people who are very helpful... thanks everyone.

Richard Bevan, for his great Saab Audio pages. 

I also found a great site to help you understand car audio... LOTS of great info here. 

Update:

I receieved an email recently asking for more objective opinions on the sound, imaging, etc... so here it is.

After living with the system for six months now, there is no question I need an external amp, as I thought.  I have just enough volume when the top is up for nice clean audio... but when the top is down, there just isn't enough power.   My plan is to eventually add an external four channel amp... and possibly a sub in the trunk.  The Infinity powered boxes look good and easy to mount. 

Other issues:  I think the location of the tweeters in the dash is going to have to change in the not too distant future.  The imaging isn't as good as it could be as a result of the audio from the tweeters reflecting off the windshield.  When the weather gets better I plan to move the tweeters from inside the dash to the bottom of the dash, right near the door mounted speakers.  That will take care of the high end. 

The door speakers are doing pretty well for the midrange... the three band EQ on the Nakamichi is helpful in getting the midrange correct. 

The low end coming out of the rear JLAudio rears is good, but the problem is the lack of power in the head unit.  I can't really turn up the rears where I want them to be without running out of amp headroom.  I did not install a low pass filter for the rears, so they are still receiving a full range signal... definitely not ideal.  One other note, there is a vibration that I haven't had the chance to chase down inside the rear driver's side panel.  Again, when the weather gets better I will take care of it.

The big question is where to install the amp when the time arises... I don't really want to put it in the trunk and run all that wire, but that my be my only option.  I suppose I could install it under the passenger side seat, I'll have to see if I can find one that fits  I'll update this as soon as I change anything.  Feel free to email me if you have any questions or advice for where to install the amp...