Ground loop. Check amp and deck ground. Run the shortest possible ground to the deck and a SEPERATE one to the AMP. Make sure AMPs ground and positive are same size. If you have an unregulated power supply it will make a difference.
Sometimes you are a simpleton.Just my cheap ass, personal oppinion, am not recommending this in the least, do not take my word for it...your results may greatly vary!
I have been using the same skinny black RCA cables fro
wal-mart , in 2 different vehicles, and I got them about 5 years ago..I don't have feedback, or at least none that I can tell, my system sounds great to me, I just wish I had better door speakers with some highs,
I ran my power down the passenger side due to where I decided to mount my amp in the first install, my RCA and remote run down the center, my amp ground is one of the nuts for the tail light mount to the body, my HU is grounded by the ground wire supplied and from the HU itself to the dash framework
I think my power and amp ground are both 8 gauge
that's what I did..cheap, simple, works for me.