Electrical weirdness with my '78 280z 2+2

[evilpete]

280z newbie here. :stupid:

Here's the deal: I noticed that every time I stepped on my brakes, my stereo speakers made a funny noise. Then I noticed that my voltage meter would sputter with my engine. It didn't affect overall performance; it was a small issue. Then, every time I stepped on my brakes, my stereo would simply shut off, or my brake light would come on. The red charge light comes on now and she backfires/sputters and loses all kinds of power going up hills. I'm having issues that seem similar to injector problems. However, I'm inclined to think it an electrical problem since the brake light and car sputtering were the first signs of this issue.

This morning I had to jump her off and she progressively ran worse. I think the battery is fine, because all of my interior dash lights and warning tone are working. I brought her home and she's parked waiting for a solution.

Do I have an alternator problem? ECU? Fuses?

I've got a stupid question: I installed new speakers in my car, could they be taking too much juice?

Help me Obi Wan, you're my only hope.

[evilpete]

She's also running rich. Since my last tank of gas, fuel economy has worsened. Will an alternator do this too?