Some people advise grounding the deck at the amp ground as a rule. Although the length is a conundrum.
Trial and error always works best. Find the best spot whether 6" or 6'away I think.

Maybe turn the amp gain down a bit also.

Good luck!

 

thats a really good theory until you plug the antenna in

 

^why so pinhead? Just asking.

 

your deck also see's the ground of your antenna