i recently installed the GM cd radio in place of the aftermarket that was there using the provided adapter kit. my car has the subwoofer control in the overhead. i couldnt notice any action from the sub. was it designed only to work with the original cassete head unit? or should any other HU provide the output for it.

I don’t have the diagram. But from memory you need to tap power into the GM plug. The amp was Fiero only and it needs spliced in.

No, it should work. The original subwoofer is meant to go inline of the factory radio... between the factory wiring harness for the speakers, and the radio itself. This is the subwoofer wiring...




The adaptor screws into the middle post behind the dash.

There could be a couple of things...

1 - Check to make sure Performance Sound (boost slider) is turned all the way up.
2 - Confirm that the speaker wire coming from the subwoofer is actually connected.
3 - Confirm that the amplifier for the subwoofer is present (it's located in the passenger side inner foot well, attached to the inside of the plastic piece).


This is everything included in the subwoofer system...

thanks, will check on all that stuff

You have the factory subwoofer amp, right?

The bottom line - the factory subwoofer amp has eight inputs that are designed to connect directly to the HIGH-LEVEL speaker outputs of a head unit.

If you have the factory service manual, the radio wiring and subwoofer is on page 8A-150-0


If you're having problem with the factory subwoofer system...

The Fiero Subwoofer System by Oliver Scholz
http://www.fieros.de/en/articles/sub.html

------------------
Original Owner of a Silver '88 GT
Under 'Production Refurbishment' @ 136k Miles

[This message has been edited by Vintage-Nut (edited 08-15-2025).]

It may be due to mine being an 85. They never had a sub. I had to slice it to power.

Great info guys Very helpful

I have a monsoon head unit and factory sub enclosure that I will be trying to connect. Factory amp is missing. Trying to find something compatible to drive it...

quote I have a monsoon head unit and factory sub enclosure that I will be trying to connect. Factory amp is missing. Trying to find something compatible to drive it...

Car Mini-Monoblock Amps are uncommon these days; check motorcycle mono amplifiers like Kenwood KAC-M3001 {dimensions: 6-7/8"W x 1-13/16"H x 3-7/8"D} but it doesn't have a remote gain control ...

Look for physical size, high-level inputs, variable low-pass filter AND remote gain control as the “Performance Sound" gain slider assembly on the overhead console won't work with the 'new' amp without modifying it....

The 'smallest' mono-amps are around 100-watts / 150-watts @ 4 ohm {you really only need 20-watts}

------------------
Original Owner of a Silver '88 GT
Under 'Production Refurbishment' @ 136k Miles

[This message has been edited by Vintage-Nut (edited 08-16-2025).]

Thank you. Trying to find a compatible amp now.. not having much luck- but I don't really know what I am doing. Agreed, to drive that one sub, 40W should be plenty

quote Trying to find a compatible amp now.. not having much luck- but I don't really know what I am doing.

Do you're attempting to mount the 'new' mono-amp in the factory location? If so, yes - it will be a challenge...

Otherwise, any mono-amp {as mentioned above} will work.

EDIT
Years ago, most mono-amps were class A/B - like SoundStream P1.210 Monoblock with very low Total Harmonic Distortion plus Noise (THD+N)

Now, majority of mono-amps are class D, which makes less heat, but normally higher distortion

Sonic Electronix shows hundreds {400+} monoblock amps to choose

If you rather buy an A/B mono-amp similar to the factory sub-amp, they have a few under 250-watts RMS @ 4 ohms

If you want 'tiny' - look at the 'Mini Car Amp' section...

https://www.sonicelectronix...-car-amplifiers.html

[This message has been edited by Vintage-Nut (edited 08-18-2025).]