August 2003: Since I now have a Gen 4 Mark 2 F-body, I can add some new bits here. The speakers in the Mk2 are of a normal impedance, they run at 4 ohms, not the 10 ohms as in the Mk1 series car (which also may have been changed in perhaps 97, before the body and engine changes). I have also been told that the 6" "woofers" (I still have to quote the woofer there, since its borderline small) are of a dual voice coil design in the Mk2. I have not yet done work to the stereo myself. Suffice it to say that finding a dual voice coil 6x9 speaker will probably be impossible. Many regular subs are available as DVC, but I doubt any of them will fit in the b-pillar location.
Location of the Mk2 power booster: above wheel well, passenger trunk. Taking off the cover for the spare and jack will let you see it. The new monsoon is quite a bit better than the old booster unit. As stated here and I think seen elsewhere, you could just put new speakers without replacing the booster unit. Replacing the booster unit seems to be an operation similar to what I describe below, in that you have to take the speaker level signal and get it into an amp.
About the Mark 1s:
The GM design for the speaker system is rather peculiar. It uses 6 or 10 ohm speakers (I think, I haven't looked lately), powered by the head unit itself and a "power booster" (what I will refer to as the booster). The stock 10 speaker system uses this, but I don't know if the 6 speaker system does or not. Also the Monsoon system is nearly the same kind of unit, but from what I am told, has twice the number of outputs to the b-pillar speaker (the 6 inch "woofer").
The flow of power is like this: The head unit drives the speakers in the door (the fronts) and the speakers in the rear behind the rear seat. The booster powers only the center bass speakers. Now here is the tricky part. The booster is given its signal by speaker level wires that are duplicates of the wires going to the front and rear speakers. The booster then amplifies the signal and sends it to the center speakers. In the Monsoon I am told the center speakers are dual voice coil, with 2 sets of wires routed to each speaker.
If you want to add an amp or new speakers you have a couple choices. 1. You can gut the whole system and start fresh. The upside is you can get whatever you want however you want it. Power, boom, precision. The downside is you may lose your steering wheel controls and have a head unit that looks odd for its size and or has the image of an old Nintendo game with lots of hideos blinking light. (Yes take note I am a fan of "stealth" technology and the look of OEM units, not to mention they have a nice tactile feel.) 2. You can of course leave things as they are. 3. You can simply replace the center speakers for more bass response. I found just new 6x9 speakers really gave a nice improvement using the stock booster. 4. You can get a new amp for the center bass speakers, and bypass the booster. 5. You could also (have never thought of this one) get a new amp and new speakers for all the speakers.
I chose option 4, new bass speakers and amp.
The Stereo Upgrade
I wanted to put new woofer speakers and amp in my stock (10 speaker) system. In the b-pillars by the rear seats is a 6" "woofer" speaker. This is fed by a small power booster that takes the signal from both front and rear speakers and amplifies it. But it is only a bass speaker, with cutoff I would guess at about 200Hz. This is located on the passenger side by the bottom seat cushion. Be sure to read to the bottom and "The Problem" before going ahead with these procedures!
My plan was to replace these speaker with 6x9s. This was extremely easy. Any 6x9 should fit. All you need is some basic tools like a drill and bits, plus a metal snipper. Take out the old 6" speaker, and the flashing around it. If I remember correctly, there were some sheetmetal screw holders, so keep these if there are. The 6x9 will probably have to be mounted at a slight angle. Try to guesstimate how deep your speaker is or measure to make sure there is enough room behind the hole that the speaker won't touch the outer body. Make a scribe line, or draw, and drill holes for mounting, and use the snips to cut out the extra space needed. Once you do this, you will be committed to 6x9s for the rest of your car's life, but I think that they sound much better. They can be connected to the stock power booster and work fine in that way. I used this set up for 3 years.
The amp was more difficult, because GM's funky wiring and stereo configuration does not make it easy to drop in components like a stereo shop would install, using RCA connectors and the like. My main goal was to keep the stock head unit. It is not the best stereo, but I love the controls, the mute function, the steering wheel buttons, everything. Keeping this means that I had to use the output from it to go to the amp (high -level or speaker level signal). My amp is a Rockford Fosgate Punch 2.6x. It has both RCA and speaker level inputs.
I ran the power cable directly from the battery, with inline fuse. The ground I had mounted to a hole I drilled in the flange of the wheel well by the spare tire. The actual mounting of the amp is vertically, strapped to the antenna mast, and one corner also on the grounding hole. It is a weird place, but it is out of the way and pretty much invisible.
Wiring the sound cables is the hardest part. You will have to cut several wires that lead into the stock power booster, or tap into them. Here is the wiring plug diagram from my shop manual that shows the location of each lead. (See below). The pink wire is the remote lead, and that goes to the amp remote line. The left and right speaker negative lines are not even required for my amp's input. Yours may or may not be the same. Cut or splice into the rear positive feed wires (I think you could do the fronts if you wanted, but that might make it weird for front/rear fading). Run these to the L/R speaker level input to your amp. Run wires from the amp speaker output to your new 6x9's and voila, ALMOST done.
A note: if you want to add an amp to your system, you must have the new amp in the same position. The system must be: Head unit > Amp > Speakers. You cannot add the amp after the power booster. This boosted signal will probably be too much for an amp speaker level input. Do not: Head unit > booster > Amp > Speakers.
The Trans Am and Firebird (with v-8) have a very high power ignition system, which generates a huge amount of noise in the whole car. On a 97 WS-6 I drove, I could hear engine whine in the speakers when the stereo was off. (I think).
The Problem: Engine Noise
The main problem that I had, and you will probably have at this point, is that there will be engine noise in the speaker when you have the engine on. I have asked a number of stereo installers what the problem is, and I get the same response. Find a better ground. Bullshit I say (and so have some others too). I have tried to power up my amp in many configuration with many grounds, and with noise suppressing blocks and the like. There is always going to be noise with the setup I have just outlined. The amplification of the speaker level signal again in the new amp makes it all the worse. The way to eliminate that is to make your speakers low-pass, or bass only. My amp had a special card in it that lets me make it full range, high pass, or low pass. It is set for 100Hz low pass, but I think I would like to make it about 200Hz. There are several ways to set your speakers for low-pass, and I think there are in-line filters you can use or use a passive cross-over box to do the job.
If you have a later year car, or one with the Monsoon stereo, your configuration may be slightly different. But I believe even in the Monsoon stereo, there is a power booster that has the same type of leads going to it, located nearer to the spare tire behind the plastic panelling.
I hope this takes out any confusion that you may have had with putting an amp in your mostly stock TA. My sounds much better now, and I can crank it up all the way without losing any bass or power.
The power booster wiring block.
You will probably use 9 and 10 (dk blue and brown) for your speaker feeds, pink for remote signal.