A whole host of electrical gremilns. Help?

[LS240]

So my S13 has a bunch of little electrical issues, all inherited from previous owners. Basically, my stereo doesn't work, headlights won't go down, guages don't have illumination, and the HUD doesn't work.

The stereo is an aftermarket Alpine unit, and it has power and functions fully on the face, there's just no sound coming out of the speakers. Every once in a while it will work for 10 seconds or so before going out again, and while it's working it sounds fine. Strangely though, sometimes when I fiddle with the headlight up/down button or the guage illumination level switch, it'll trigger said temporary functonality.

The headlights are stuck up, and after manually lowering the headlights I know for a fact it's not a motor issue as they pop right up on command. They just won't go back down. The relays all seem fine, unless we're not testing them properly.

The guages just don't have any backlighting at all, and haven't since I bought the car. The clock and little red warning lights work and that's it. The HUD simply doesn't function.

My Father and I have spent most of today trying to chase these issues with the help(if you can call it that) of a Chilton manual, but so far haven't gotten to the root of the issue. We'll tear back into it tomorrow, but I figured I'd ask here to see if someone could lead us in the right direction. The stereo issue really just seems like a short, but the fact that messing with switches that would normally control the other functions that just happen to be not working makes me think they're somehow connected. Could whomever installed this stereo simply have tied into a power source they shouldn't have or something like that?

I'm at a loss because wiring isn't exactly my strong suit so any help would be greatly appreciated.

Thanks for taking the time to read.
-Layne