it's a ground problem. Try grounding the chassis of the deck as well as using the ground on the harness.

as a last resort, you COULD use a ground loop isolator, but they are just a bandaid...try to find the problem first.

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http://www.sounddomain.com/ride/818309

My old van and install.

http://www.sounddomain.com/ride/634352

The rest of the install pics.