I've seen a few threads on this but I thought I would post up my swap. I did it in about an 1/2 an hour last night...
Having owned 5 f-bodies (three 4th gens) I've collected a few spare parts. One of which was a Monsoon sterio from a 00 Trans Am. The radio in my new 88 GT has a ghost or something as the volume would randomly go up or down for no apparent reason. That and neither of the right speakers worked. Weird but anyway, I decided to swap it out.
Removing the stock Fiero radio was a pita because of the little pushlock on the back of it. The new radios just use a dowel. I used a long flathead to push pop it loose and the radio came right out. Once the radio was out I just disconnected the wiring harness and did my best shot put impression with it.
For the adapter harness, there are a couple of solutions. I took the more expensive but quicker solution which was to go to the local sound shop and buy two connecter harnesses to build the conversion harness from. Metra makes a few harnesses that will plug into the back of a stock newer model GM radio and into the fiero wiring harness. I've seen these somewhere on this site for like 40 bucks but you can get them for 15 here ( http://www.audiooutfitter.com/store/cats/270/harnesses.html ). The plug used to convert the Monsoon radios in the f-bodies to the stock Fiero harness is the Metra part # 70-1862 harness found here ( http://www.audiooutfitter.com/store/701862.html ). There are a few other 12-21 pin conversion harnesses with various connectors on them listed on the first link.
Because I couldn't find the adapter harness locally I built my own. I found a couple Metra harnesses, one that plugs into the newer radio and another that connects to the harness. They cost me 25 bucks from one of the local MASSIVE MARK UP stores. I just soldered the two together and heatshrinked them. Works like a champ. I believe the Metra part number that plugs into the back of the Monsoon radio is 71-1858 but the one for the fiero escapes me. I do know I found a harness that plugs into the stock fiero harness at Walmart for less than 7 dollars. You will also need a 4 dollar Metra AADP4 antenna adapter. These can also be found at Walmart under a different brand.
To reintall, the dowel pin on the back of the new radio lines up with the support in Fiero. It took a little pushing and clearing the wires but it went right in. Also on mine, only the top two holes lined up with the stock fiero brackets. Between those and the dowel in the back, I think that is fine. I couldn't make the CD player skip. The face plate now bows out a little bit so I will have to play with it and figure out why. It might have always been like that and I hadn't noticed. Anyway, I hope this helps!
Stock radio from a 2000 trans am - 80 dollars
Wiring conversion - 30 dollars
Having a decent stereo that matches the interior and lights of your car - PRICELESS
