current use such as brake lights = temp/fan and shift light come on?
 

When I'm idling at a stop light with my foot on the brake at night with my headlights on, my shift light and my fan/temp light come on. If I put on my high beams at an idle, they'll flicker. There's a weird buzzing sound associated with the flickering, but no sound once the lights are on for sure. The other night I could get them to come on if I pulled up on both window buttons.

My car is an 87 TII with an FD alternator. Let me say that at idle, my voltage has always dropped to between 12 and 13 volts. If I so much as touch the gas, it goes up to 14+ where it should be. It's been like this for years. I've never seen it put out a good voltage at idle. I haven't tested these in a long time but I just know that it's always been like that. I added many ground wires in an effort to eliminate this, none of which seemed to do anything.

After not driving my car much for a while - I drove it enough to keep it in decent shape, but didn't go far due to it not being inspected - I just took a road trip of about 1300 miles during which time I used, at idle, one of those emergency kit cigarette lighter powered air compressors to keep my old tires inflated. I also have had the negative battery cable finger tight because something was draining it so I would disconnect it if I was going to let it sit for more than a night. I'm wondering if all this could have fried my nice FD alt, or if there's something else weird going on...

I should also say that nothing weird happens if I'm not idling. Even if I'm slightly above an idle, everything is fine.

Well, it'll take an electrical wizard like Hailers (or his Mini-Me, Hailers2) to untangle the connection between the shift light/coolant light, etc. but it definitely seems like your alt isn't wired properly.
My FD unit never varies from 14+v, no matter the load, no matter the RPM.
If you got that straightened out, I'll bet at least some of your problems would go away.

I'm pretty darn sure I have the alternator wired properly. It is wired just like this image says:

https://www.rx7club.com/attachment.p...5&d=1203426343

The only thing it doesn't explain is what to do with the B/W "ignition switch" wire that went to the S4 alt but for which there is no connection on the FD alt. I just left that one unconnected. I can't remember if I found something that said I should do that or if I just didn't find an answer for that wire... but the others are correct.

I do remember having trouble finding a good size belt for it. It seems to be at the very end of the adjuster bar and still is not extraordinarily tight. Perhaps it's loosened up... but I doubt that's the issue, since it's not slipping or anything. There definitely seems to be some kind of electrical "decision" occurring at a certain RPM.