bad ground

I have a 96 Jetta Trek with a stock system as far as I know. I had a low grade JVC head unit in that I wired myself and worked fine for about 2 years. While driving the 2 front and 2 rear speakers cut out but my subs and amp in the trunk would still work. After cutting power (the main plug to...

So there is engine noise in my rear speakers and not in the front all ran off the same amp if I switch it to 2ch it stops the engine noise and I don't think it's the amps ground because I have my sub amp grounded to the same bolt on opposite side of car any ideas where to start