the signal ground is from the HU, where it is grounded, to the power amp where it is grounded (at the chipset in your case) if either is weak you get noise, also if any of the solder points in the signal path are weak you can get noise (also if the RCA itself is damaged). I dont think a RCA to case ground will hurt you.

Usually a ground loop is by poor component grounding so verify your amps negative has a solid and tight ground point.

 

hah i knew it was the grounds... so ya pull out your deck real quick and check the grounds to it...
its so easy to take out a deck..

 

I'm not sure where I put the keys.. haha..

 

lol.. did u put em in the box your deck came in??