By "feedback" do you mean a hum? And is it when the car is running?
First things first, check ALL your earth points - ie. make sure the amp is properly earthed, the h/u is properly earthed, and the earth points for both the battery to chassis and engine.
Well do it once do it right i say , Bigger earth wire wouldnt hurt, 8G a bit small and find a decent earth, if its a painted part of the chassie, remove the paint under the earthing point too.