Pre Outs Gone Bad?

[MountaineerGreen]

For starters, I have had many different systems in the past. I have wired up lots and lots of head units, amps and subs. I am no rookie to the car audio thing, but this one has me stumped.

Specs:
Pioneer DEH-P6500 head unit- has pre amp output for subs
New Dual 800w mono block class D amp- 200w RMS @ 4 ohm
cheap a$$ Lightning Audio 10" 4 ohm 750w max sub
Custom box I built to fit in my cargo hole ~ .7 cubic foot air space- not ported

What I have is noise through my sub anytime the HU is on and the sub setting is on. The noise is engine, the internal CD motors of the HU. I also have a pronounced thunk when I turn the truck off. All this is when the RCA's are connected to the sub output.

I can connect the RCA's to the rear output which is post amp (I believe) and everything works fine. No engine noise or thunk. The ground wire on the amp is about 4" long and has a solid connection. The amp is mounted to the inner structure of the body behind the drivers side rear cargo area. (jack compartment)

I went without any amp or sub for a while, but all of the wiring was in place. It is possible that the RCA's touched something during that time. I don't know whether that would mess up the pre outs or not. I have used the pre outs before and they worked fine.

I plan to ground the chassis of the HU tomorrow and see if that helps. Does anyone have any ideas? Maybe a noise filter?

 

[FexplorerV8]

I think its the RCA cable, most likely its touching ground somewhere along the way pull it out and make sure it has no cuts.

 

[MountaineerGreen]

Well, I talked to a Pioneer dealer today. The guy said that if my RCA wires touched any metal when connected to the head unit, it could mess up the pre outs. I'm almost certain that they touched the body of my truck. I'm looking into replacing the head unit either via ebay or getting the unit repaired. Just depends on what the costs are.