98 sunfire se stereo problems

I have a 98 sunfire se and I put a JVC head unit in my car and I'm pretty sure the ground wire is bad from the unit to the chassis because when I turn the volume up past a certain level it dims and most of the time just shuts off. The headlights dim sometimes when I push on the brake also. I was wondering where exactly the ground wire is and how do I get to it? If you could help me that would be awesome cause I have 2 12's and an amp I still have to put in and I can't do it until I figure out this problem.

It doesn't sound like a bad ground its sounds as though your alternators not putting out enough, or something is drawing too much. It is too much of a coincidence for your brake switch to be dimming the lights. If a deck is underpowered it will shut down, and as you increase the volume it has more demand. Also if you find it is not you alternator and it is in the unit , just run the black ground wire off the deck directly to any bare metal that is grounded well. Although i doubt this. Also you may want to meter your speakers to see if they are not blowin causing the deck to go into protection mode.