I pulled the instrument cluster to replace the bulbs and noticed a heavy guage black wire attached by a spade connector to the back of the fuse box (opposite the side where the fuses are). It is disconnect, and I don't know if I accidently disconnected it while pulling the dash or not (looks like someone might have pulled it years ago). I can't seem to trace where it goes. Does anyone know what this wire is/does, and should I hook it back up?
Thanks!

Check for two wires on the amp gauge.

Okay, I'll try tracing that back to the gauge. Any other ideas?

Dunno the year we are talking BUT only spade terminals should arrive to back of fuse box should it be with RED wire to the Batt buss bar ( devices feeded directly from batt/alt, and not ign key ) and BLACK wire, being the ACC buss bar, and comes from ignition switch to feed accesories.

both are as far I remember 12 gauge

So probably you unplugged that wire.

you can check if you have power there constant or just when put the ign key on accesories position

Thanks! I'll plug it back in and see what happens.

well thats easy... if you have TWO buss bars, both must have a wire to get power. If not, then you have a fuses side getting not juice