ls1 problem, please help

I just bought a 1999 Z28 automatic. My lights (all of them) flicker like crazy and my volt gauge jumps around, it usually stays at about 3/4 up. If i turn the defrost on, it stops, if i turn the ac on full blast it stops. I also noticed that it flickers worse when i push the brakes. Also my security light stays on all the time. The guy i bought it from had a system and took it, now only the way back speakers work and the little light by the power button doesnt light up. Any suggestions would be appreciated...PLEASE HELP ME!!!Thanks

...sounds like a bad ground somewhere. Check the battery...there should be wire coming from the negative terminal and grounding near the shock tower. I think stock, it's a 10 guage wire or so. I'm thinking maybe he took a shortcut when he disconnected everything.