No amps with the rear speakers. Just the OEM factory 10 W (!) 6" dual cone speakers, one in each shock tower. There are two wiring harnesses in each shock tower; one harness isn't connected to anything, the one that is connected to the speaker has three "pins"/fins, only two of them connect directly to the speaker and the third doesn't seem to be used. A continuity check with my voltmeter showed the black wire had a low/almost no resistance to ground (on a metal bolt handy in the tower) so that made sense that black was common and it was nearly open circuit (few hundred megohms) to the other two pins in the harness.
I will check the fuses in the box near the clutch. But I do have power to the headunit still so unless there is a separate fuse for the amp for the rear speakers located with/behind the stereo stack, the fuse is likely not the issue. I have the factory Clarion radio with the OEM/factory tape deck and that is all, ever since 1986 and has been working fine throughout (amazingly). Am not happy I messed up a good setup for no good reason...

 

Based on your description, according to the service manual, you have System 3 that uses a Type 2 headunit, which has a separate power amp for the rear speakers located inside the center stack below the cassette player (it's a small silver box). Before you go replacing the amplifier, you can try connecting a speaker directly to the outputs of the speaker amp.

The connector in the rear speaker well that connects to nothing is the power and ground for the optional rear speaker amplifier if your system was the System 2.

I don't think you ruined it. You might simply have missed something simple. You've already eliminated the speakers as the issue and the front speakers are working.

The fuses for the sound system should only in that interior fuse block. Check them first. There should be only two fuses in that panel that need to be checked. Look for the fuse marked Audio 20A (it might vary) and power antenna. You may want to check clock/warning as well just to be thorough.

Since it's been a minute for me, I went into the garage and cracked open the 1988 Owner's manual from my glove box and... I didn't find a list of interior fuses. So you have to go by the fuse cover on the interior fuse box.

One thing you can do that might keep the brittle plastic from breaking is soaking it in interior cleaner--one that's safe for plastic-- which could help keep the plastic from breaking. To remove the center stereo/ac panel, you pull the shifter panel up (it snaps into place) and pull it aside, remove the ashtray, and carefully pull out the center A/C vent. Then, remove four screws and the panel will come away (it's not snap -in) and unplug the cigarette lighter and lamp connectors. You can use a plastic trim removal tool on the shift cover and on the edges of the AC vent panel (I forgot the S4 panel is a different design from the S5). Go slowly if you decide to go that route.

 

thanks. I have removed the center panel before per that exact process and steps. And although expensive they still sell that piece (and the center vent piece; both for an outrageous but not unexpected price..) so if worst comes to worst, can still replace it without custom 3-D printing the part myself. I just checked and some good news is that there is power (about 12 V dc) between the blue/white and black pins to both rear wiring harnesses and only when the radio is on/key is in the ignition on acc etc. So will check the connections on the rear wiring harnesses themselves, maybe they got messed up to the spade connectors or something (that would be the best outcome of course). Speakers seem good; resistance is about 3.6 ohm for both the old and new speakers. Will need to get some daylight and some time to check things out further. I also found the fuse listing on the fuse box cover but doubt it is a blown fuse because I do have the radio on fine and sound through the front pair of speakers and the balance control seems to work too as does the tuner etc.

 

A lot of times, these amps need new electrolytic capacitors, as does the rest of the car. I have recapped all my s5 stuff, and some s4 stuff. If you open up your amplifier, I am willing to bet that you have some failed caps.

 

That is also a possibility and capacitors are cheap, easy to get and easy to replace if you have a soldering iron.

 

Happy ending/update: Agreed that the capacitors are generally the failure component on almost every electrical component that isn't just completely zapped with current by user error. And that was my next step. But daylight and some less panic and firm connections to the speakers revealed that it was something stupid; maybe when I pulled down the amp box below the glove compartment I did something or just didn't have the rear connections actually solidly connected. But it all works fine now in full (and sounds much better). I also learned a few things: that the 4" speakers in the front are actually not 4" speakers; the hole to hole diameter for the mounts is larger by about 1/4 to 1/2" than the mounting tabs on at least the 4" Polk speakers that I just got; had to have a custom mounting plate fabricated to get them installed both below the left dash and in the speaker box under the glove compartment. And the 6.5" speakers in the back from the factory are really only 6" speakers and 6 1/2" speakers don't fit (barely) but instead of having to make another set of custom mounting plates, a minute or so with a dremel tool did the trick and is a very easy mod to allow standard 6 1/2" speakers to be installed easily into the rear shock towers (for those vehicles that have 6" speakers there vs 4" speakers with amps; I don't know how large those actually are but wouldn't be surprised if they weren't standard 4" speakers as of current either). I also learned from scouring the boards how to properly bypass the factory amp and wire up an aftermarket head unit with a relay to keep the power antenna functioning so it won't be wasted and I will eventually get around to doing that even though it will lose the aesthetics of a purely stock 40 year old S4 gem if I do. Thanks for all of the help and super prompt replies as well!

 

I have an early model 86 and my stereo was different me and another user found the following guide on reddit:

CASSETTE BELTS - Pennock's Fiero Forum

i ended up using a med sq 8.5 or 8.0 i can't remember now, but that's what i used.