I believe the Oliver Scholtz wrote that schematic ten years ago for an article on how he repaired his OEM sub woofer amplifier. If you go to his website he's added many modern electrical features to his Fiero. The sub-woofer wasn't a common option and very few factory replacement parts were ever available. Like the radios, it could be possible that other GM models used the same amplifier
" THE BLACK PARALYZER" -87GT 3800SC Series III engine, custom ZZP /Frozen Boost Intercooler setup, 3.4" Pulley, Northstar TB, LS1 MAF, 3" Spintech/Hedman Exhaust, P-log Manifold, Autolite 104's, MSD wires, Custom CAI, 4T65eHD w. custom axles, Champion Radiator, S10 Brake Booster, HP Tuners VCM Suite.
87GT - ALL OUT 3.4L Turbocharged engine, Garrett Hybrid Turbo, MSD ign., modified TH125H
" ON THE LOOSE WITHOUT THE JUICE "