Battery light on but not all the time.

Hello, so I've been having this problem for
a while now. I can't recall when the
problem started. Basically my car will start
perfectly fine and drive fine also. I have a
aftermarket stereo in my car that my uncle
installed for me. The battery problem may
have started after he installed the stereo. I
did realize that there was a bare wire once I
took the stereo out of the car, but I took
out the aftermarket stereo and wire
harness and hooked up the original stereo
and drove on it for a few days and the
battery light still comes on. When I'm
driving there isn't a light at all but once I
come to a stop it slowly starts to dim. If I
roll down my window it gets brighter. When
I turn my head lights on the light turns on
also. I also found something weird out. If I
turn my stereo up loud it gets brighter
when bass hits then dim when there's no
bass. If anyone knows what the problem is
or what I should do to trouble shoot it then
that would be awesome.