Bad speaker whine

[brandonros]

Hi,
I recently bought an 04 Yukon xl and I'm loving it but it had several little issues.
One of them is that the rear left speakers have a horribly loud whine that is present when the radio is on. It doesn't matter if engine is running or not. The volume of this whine doesn't get louder. It stays a constant loud volume.
One thing I've found is that if I have the ignition turned to on without engine running, then I dim the dash panel lights all the way to off, the sound goes away and both front and rear stereo head units have no display but still play music fine with no whine.

I've tried putting dielectric grease on the amp contacts and rear head unit contacts but it didn't help. I've found that the battery level is good and the battery chassis ground is tight. I'm not sure where else to look. I've wanted to check the amp's ground but have no idea where it's grounded. It looks like it gets its ground from the head unit. Any ideas?
Thanks

 

[brandonros]

Anyone?

 

[YukonXL04]

Check all of your connections, The battery positive and negatives. It sounds like your getting interference. Also have you checked the connection on the speaker itself?

 

[brandonros]

Yeah, I've checked these speakers with other stereos and they work fine. I also disassembled the amp and rear head unit and tested capacitors and checked solder points and everything looks good. The problem changes with lighting changes though. Does anyone know where the stereo and amp might be actually grounded? Behind the dash or under the hood?

 

[brandonros]

Hi,
I'm still looking for help with this. Would anyone know where the ground points are on the vehicle for the sound system? And any ideas on how to troubleshhot and determine if this is caused by bad head unit, wiring, or amp? I tested all speakers amd they're all good.
Thanks