As requested, I put together some pictures and part numbers for 95-99 Nissan Maxima and Infiniti I30 cars that use the same type of Bose amps as the C4 does. They are functionally swappable, but the exact speaker tuning may be slightly different. I found that they sound great in the C4 as they have a similar small/large front to rear (respectively) speaker arrangement.
History- At first I set off to buy a replacement amp for one in the rear that squealed. Once I got that working with a good used Corvette unit (that I parted with $50 for), I realized other amps were weak and another completely dead- I think I had one good one in the whole car. You just couldn't tell because the squealing one was very loud. After reading up on the how-to to replace the capacitors, I redid mine (with used caps from some junkers I procured). Not using new caps was a mistake, but I got 2 out of the 3 working again. One of them failed three weeks later, but I did realize that some would be serviceable. For awhile I was trying to keep it proper using exact oem parts like an NCRS collector would do, but after having taken apart the enclosures a couple times too many, and having spent money for little gain, I considered the information I had found on the Nissans that used this same amplifier.
Nissan front amp- marked 'Sigma 975':
front amp opened:
rear amp:
rear amp side-by-side with oem C4 amp:
note on the last picture that some of the electrolytic caps appear missing on the Nissan part, but they are still there, just in different locations as the board had a layout revision. They work and I was hard pressed to hear a difference. You can tell bass/treble tuning if you swap a front to the rear and visa-versa so it is important to get that straight, but they are a good match for the vettes IMO.
I have been able to get mine from a local pull-it-yourself dismantler for $7 each. They are newer than the ones in our cars, and few people care to restore maximas, so this is the prime age for finding tons of these cars in the scrap yard. I have been able to find at least 4 amps each time I visit my local spot. (No, I have not been buying more).