Note - these instructions have not been reviewed/formatted yet. (in other words,
Adam didn't write this ;-)
The '90 models with 5-speaker systems didn't have the rear connector on the back of the head unit for audio input, so you must purchase an FM modulated changer to get it to work. The thought of FM modulating a CD signal makes most people cringe, but in my research, I found that the FM method actually works quite well if you buy the right model. I've heard that many Sony FM systems are inevitably muddy sounding, and a hopeless cause overall, but I have had no direct experience with them. Instead, I bought an Alpine 651RF 6-disc changer for only $399. It has very crisp, clear sound, and I challenge anyone to prove to me that the built-in factory disc player sounds ANY better. It comes with a thin remote, the changer itself, one magazine cartridge, a controller box (power supply and RF modulator), LCD display panel, and the mounting hardware. The only other thing you need to purchase is an antenna adapter because the Bose unit uses a funky non-standard plug. I found one at the local car stereo shop for $19.
Where you mount the changer is mostly a matter of taste. It has been reported that the Alpine changer (the physically smallest one made) will fit perfectly under the leather passenger seat. However, I installed mine there and had problems with the seat scraping the box, and sometime the box would cause the seat adjustment lever to not lock properly. But, I have cloth seats, and I bet that is why I had problems. Another option is to put it on the rear deck, between the passenger seats. I haven't tried this, but when I move my changer from my NA to my TT, I think that's where it's going to go.
To install the unit under the seat, you must first remove the seat. It's not hard.. just remove the four bolts holding the seat frame. In fact, this is a major bonus: thieves won't know you have a changer, and even if they do, they won't be able to steal it w/out first pulling out the seat. The mounting of the unit itself is flexible. You can bolt it directly to the floorpan of the car if you like, but the thought of corrosion made me shy away from that option. The other choice is to use the four plastic plates that come with it. Each plate has a bolt fitted in it, facing upwards. What you do is you make a cut in the floor carpeting and slide the plate underneath. Then, you poke the upward facing bolt through the carpet and use this as a mounting point for the changer (putting a nut on top). I found it easiest to make the cuts right at the points where the carpet was squashed down by the seat frame. Then, I lifted the carpet and slid the plastic plates under and positioned them to line up with where I wanted the changer. Next, I bolted the changer down and fed the DIN cable up the side of the console wall. With a little messing around, I managed to route the cable inside the console wall, all the way up into the dash. I then put the passenger seat back in. Be careful not to scrape the door rail paint with the seat frame brackets when moving the seat in and out - the seat is heavy, and it's easy to slip.
Okay, now you need to just hook up the controller box and find a nice hiding place for it under the dash. This is really hard to do because due to cable lengths, the controller box must be near the head unit, and there's not much room under the dash. This is the most sloppy part of my installation. I ended up finding a spot deep up under the dash (on the driver's side), near some vent ducting, but I couldn't get it to stay up there. Under heavy braking, it kept falling down onto my feet (not a safe situation!) I gave up and used some duct tape to hold it in and it worked great. This is also a good location because the fuse box is nearby, and you'll need constant and switched power taps from there.
Next, you have to pull apart the dash and remove the head unit. The antenna adapter fits up easily into the rear of the head unit, and connects to a DIN cable coming from the controller box.
The last thing to do is to plug the LCD display into the controller box and find a place to mount it. If you have the 4-speaker system, and thus the 'cubby' hole under the head unit, then you're in luck. The cubby makes for a very stealthy mounting spot. You simply cut out a hole in the rear of the cubby, big enough to pass the DIN cable through, and place the LCD deep in the back of the compartment. With this setup, the only way to see the LCD is from a sitting position. From the outside of the vehicle, your changer is 100% hidden. If you have the 5-speaker system, your choices are somewhat limited. I decided to mount the LCD under the climate control pod, with velcro attached to the cloth part of the dash. I had to superglue the velcro to get it to stick properly, btw. But this location is actually quite nice and fairly hidden from window-peeeker's views. If you place it properly, the panel fits in very snugly, pressed into the upper right corner. Then, you have to route the wire so it's snug up the the underside of the dash and doesn't hang down (not hard).
The little remote control fits perfectly in the space beside the power mirror switch on an NA. In a TT, the mirror heater (if yours has one) and suspension setting switches might get in the way.
I've found the Alpine to be a very good performer. It's cheaper than many of the competitors, and hasn't skipped ONCE since I installed it (unlike the stock player). Not only is it the smallest 6-disc player you can buy, but it also has the fastest time for changing discs. When mounted under the passenger seat, changing the magazing is accomplished by leaning the seatback forward and accessing the changer from the back (so in other words, the changer is facing backwards). Also, perhaps one of the reasons why I had problems fitting the changer under the seat was that I chose to mount it to the carpet plates, instead of directly into the floor. Maybe if you screw it directly down into the floor, you give it that tiny bit more clearance that's needed. Or, I suppose you could cut the carpet out in a square for the changer to sit in, giving even more room.