Here's a link to my photobucket. It's showing the old ground point, and the new point is highlighted in blue.

 

 

Well, the blue highlighting won't paste apparently. It's the large bolt near the bottom left of the picture.

 

Sounds like you may need a ground isolation filter on the RCA's. It the ground inside the equipment are different IE chassis or floating ground this can happen. As a experiment put a striped wire from your new grounding point to the outside of the RCA at the amp and see if it stops. If it does then grounds are different types or the RCA's have a bad ground. And a FYI, If a installer thinks a ground with a tech screw is sufficient they are a assclown. Just my 2 cents, but what do I know I've only been doing low volt for 27 years.

 

What are you using to convert the high level signal from the factory head unit to low level input for the amp? Did you buy a processor of some ind for that or are they just using cheap 25.00 line output converters? If it's the latter I would seriously consider a better product like the Rockford 360 or if that's more than you want to spend the audio control lc7i is a great product and can be had for well under 200.00. And will make your factory head unit send a true high volt pre amp to your amp.