audioholic
5,000+ posts
not a moderator
If you are describing the problem correctly, it has to be the h/u. There's no way the RCA's or anything else downstream in the signal chain could also cause the speakers being driven off the h/u to display the same hiss/buzz/whatever.
Rewire your h/u. I always bypass the factory harness completely, as Ive found too many to be 'noisy'. Its simple, just run a new power wire over to the fuse panel and plug it into the protected side of the fuse. Then run a new and dedicated ground to the nearest metal that contacts the chassis/body.
I realize you already did the ground, but as said before, a self tapping screw is far from ideal. Sand the area thoroughly, drill a larger hole, use a bolt, and use some dielectric grease when you put it together. And be sure the metal part you are using makes substantial contact with the body/chassis. Sometimes dashes have metal straps or brackets in them that only fasten to plastic, or make very poor contact with the body.
