Ground loop?

[bean-a-reano 10+ year member]

Yesterday I helped my friend install 3 MTX 15s into his Mercedes ML430. He didn't want to get an aftermarket HU so we had to doa a line converter on the rear speakers. We had a hard time finding a decent ground. Ened up using this mounting bracket for the CD changer (on the passenger side). It is probably not the best place, however he is impatient. After some tweaking the subs sounded okay, so I shut the rear passenger side door. As soon as it latched the ground noise which there was a minimal amount before trippled in volume. The door has to be completly shut for the noise to be that loud. I am sure it is a ground problem, just not sure why the door is triggering it. The other guy helping thinks it is a pinched wire. What do you guys think?

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[dkguitarist 5,000+ posts]

find a bolt that is directly to the chassis/frame. and scrape away all paint.

 

[phantom240 5,000+ posts]

couldnt be any clearer,dk.

Done.