Jonez... where is it you said that you grounded your amp? like Koz said, it needs to be grounded very close to your amp, like if you pull back the carpeting on the sides of your trunk, there are bolts and metal, that's where i grounded mine. (always remove any paint or dirt first)
I'm kinda supprised the amp has even worked, the amp i'm useing will go into protect mode if it doesn't have a sufficent ground. but connecting to a battery ground is no good. you need a large piece of body metal close to the amp.
yea its good fun grounding an amp, my amp kit only have me a 2 ft ground wire, i had to go out and buy 3 ft of 4Ga cable, that just pissed me off, i was 3in too short with the first ground wire...damn money hungry companies!