94 Cavalier Ground Problems--please help

Hi,

I have a 94 cavalier, and I have installed my own stereo, which includes:
kenwood deck
two 6 by 9 speakers in the back
two 5.5" speakers also along the bak between the 6 by 9s
two subs in the trunk
amplifier to power the sub
amplifier to power the speakers in the back

I have tried everything to get it to ground properly.. I have ran separate grounds, I have even gone under the hood and ran like 3 wires directly to the battery negative terminal from different locations from the body, and nothing has worked. I have 2 noise-surpression filters running; one on the power lines and one in-line on the RCA jacks. They work, but I can still hear the engine noises. If I take them off, it sounds like I have a "riced" muffler on my car. Does any one know how I can fix this, or where the main ground line is?

Thanks.