I've got the same thing going here guys/gals, I've got a Pioneer Premier DEH-P500UB H/U my sweet girl got it for me on my birthday, its a sweet deck.
Had it for almost 2 yrs now, warranty is almost up, and one day, it started whining after an install..Like most of you out there, I grounded and regrounded everything, removed paint, drilled new holes, even up graded Cables..Nothing seemed to work. hell I even isoliated the whole H/U...Now i more or less stumbled a crossed this thread, and I have to hand it to you guys, I freaking worked. I had every kind of buzz, whine, thump, pop, you could think of. Now Its as quiet as a sleeping baby.
Took some remote wire I had around, and grounded my RCAs, and I grounded the deck straight to the batt. Perfecto!!!!!
So, Thanks, you saved me a buttload of frustration,time and money.