Like said above most good amps have a "remote volume control" port, that you'd normally plug in that brands control, they are wired.. I haven't bought an amp in 2 years, but all mine from the mid 90's till the one I bought in 2013 used a cat type cable (think home phone)
You'd just need to get the pin out for the remote volume and match the fiero slider to it, If it is nothing more than a volume rheostat, I've never seen one so I can't tell you for sure, but my bet is it's nothing more than a slider rheostat, that said
I'd not use it the way you asked, "bridging to mono" if that ment you planed on wiring the head units rca's together, or the head units speaker level leads.. neither are made for that..
once you have the wire pin out for the remote of your amp, and know what ,what wires are in the fiero control, you can wire it up..
from what I can tell from the limited searching I just did the non performance map light unit has 2 wires going to it, ( orange and a white one) these power the map/reading/door lights in the 4 light pod,
the sub controller wiring adds a yellow and white most likely molded together . If I knew where my 87 shop manual was that I could put my fingers on it.. I'd post a photo of the wiring.. if it was in there..
below is a simple volume control wiring.. you'll note that it has 3 leads input/output/ground..
you'll be dealing with the input/output of the potentiometer (volume control)
on the slider you just have to find out what spade terminal is the input to the control and what one is the output.. when you look at the wire connector, you'll have 4 wires orange/white/yellow/white
and it should become clear what two spade terminals on the light unit is for the lights and what ones got to the control..
now you just have to test the potentiometer ,http://www.instructables.co...-of-a-Potentiometer/
you are using it as a volume control
[This message has been edited by E.Furgal (edited 02-23-2015).]