You may want to replace the wiring with some quality 14 or 18 ga wires. If you are installing an amp in the back, run power down the passenger side, ground and patch cables on driver's side, as short as possible. If necessary, cross at right angles. This will keep alternator whine at a minimum.
The following information is for the Coupe. The system in the sedan is similar, but you may have to be creative to figure out how to mount new speakers.
Front Speakers:
The front speakers in the dash can be removed from above. Simply (and carefully!) pry the grille off with a small flat-blade screwdriver. Just about any 4" speaker will fit with some minor modifications. A major problem with the front speakers is the stock grille. To improve sound quality, some owners have meticulously widened the airholes in the grilles with a needle. Others have cut the grilles off and stretched speaker cloth over the frame. Another option is to replace them with grilles from earlier Audis, which were made of actual metal mesh with cloth inside them. Or you could just use aftermarket grilles. Whatever you do, it will be a noticeable improvement.
Rear Main Speakers:
To get to the speakers, you must first remove the bottom cushion of the rear seat (2 screws), then fold down the rear seat backs and remove the side bolsters by unscrewing the striker pins with an allen wrench, then pull down on the bolster. Try to take note of the approximate position and length of the striker pin, because they are kind of tough to line up when you reinstall them. With these pieces off, you should be able to now remove the screws (4, I think) that hold the side panel on. With the screws removed, you should be able to remove the panel by lifting up.The factory plastic grille is curved to fit the panel, so it must be retained to avoid fabbing up a new grille and looking for matching material, etc. Plus, it is integral to the assembly of the factory components, so it is easiest to keep the grille and rework the bracket to accept your new speaker. The factory speaker mounts in a twist-on clamp approximately the size of an aftermarket 5 1/4" driver. You may have to cut the mounting edges of the drivers and/or rework the factory clamp, but this setup is possible.
The speaker/amplifier can be twisted out of the bracket (it's held by four flanges on the bracket). You can flatten the flanges, then position the new driver against the bracket to determine the best alignment for mounting. Now drill holes in the bracket that correspond to the holes in the driver, and use small nuts and bolts to mount the speaker to the bracket. You can now mount your new speaker on the panel using the OEM bracket and retain the curved grille.
The only difficult part of putting everything back together is getting the striker pins aligned for the rear seat back. Don't tighten them down all the way until you're sure they are positioned correctly.
Rear Tweeters:
Many people don't like the effect of tweeters in the rear of the car, so they just disconnect them. This can be done by snipping the wires that come out of the rear speakers and feed the tweeters. However, if you want to replace them with something else, here are instructions for removal:The tweeter assembly on the rear of the CQ consists of a speaker grille, a threaded housing, a threaded collar, and the tweeter. To access the assembly, you should remove the parcel shelf, pull out the spare tire, remove the rear supports for the shelf (one screw for the passenger side, by the rear wiper washer reservoir, and two screws for the left support which also supports the top of the spare tire). Next, remove the straps/collars for the rear setbelt retractors (on top of the tweeter panel). These come out by pushing them towards the outside of the car, then lifting up the front edge. In order to remove the tweeter panel, you should use a stubby philips screwdriver to remove a small philips screw that is hidden inside the opening for the seatbelt (leading to the retractor). There is also a screw where the tweeter panel meets the side panel at the base of the window. Now the fun begins...
Like for the rear main speakers in the side panel, you'll need to remove the rear lower seat cushion and side bolsters (see above). Once that is done, you should be able to gently pull out the panel that contains the tweeter. Pull out some slack in the seatbelts and use something like a clothesline pin to keep them from retracting. Looking at the tweeter from the bottom, you should be able to see a threaded plastic collar with little tabs. You want to loosen this collar to remove the tweeter (a little WD-40 may help here).
Now, to remove the factory grille from the tweeter, place the assembly into a pan of hot water to make the plastic housing pliable. Carefully work around the edges of the grille with a pin driver or other pointed strong tool (wear eye protection, folks!). The metal grille should pop off the plastic housing/tweeter. Remove the three small philips screws, and the tweeter will drop out of the housing. Now do what you must to get your new tweeters to fit...
Car Stereo Review lists an install in a 1989 Coupe Quattro as having appeared in their Sept/Oct 91 issue.
