static, popping, and other noise problem

My friend has a Dodge Intrepid (don't know exact year, like 2000 or so, that style), with an aftermarket headunit (again.. don't know model) and JL components up front, stock 6x9s in back, and a Kicker amp with 2 Type Rs for subs.

Ok, so just recently it's started acting up. When he turns the car off, the left side speakers pop, the right side doesn't appear to be popping.
It does this whether the car's engine is running or not. Also, static comes through the speakers no matter what when the radio is on. Fuzzy static, doesn't get louder as you turn it up.

The left side speakers sound bad compared to the right side. We swapped speakers side to side and they still sounded bad on the left side.

Also, when the headunit is playing a CD, the noises the CD player makes (mechanical noises.. skipping songs, spinning, etc) get amplified through the speakers. You can make it out very clearly up front, and the subs rumble around in the back.

Some things I did to try and fix it:
-Completely disconnect the amp.. removed fuse and unplugged RCAs from headunit. No change
-Grounded headunit case and added extra ground directly to the chassis. No change

The pop when it gets turned off is the same no matter where the volume is at.
The speakers aren't amped, just run right off the headunit.


That's all I can think of.

I'm at a loss guys... any ideas?? Sounds like a bad ground, but I grounded everything I could think of.
I'm ready to blame the headunit.. is this possible? Thanks.

Hummm, I think it might be, actually. Damaged shield ground would cause this, all the funny sounds through the un-amped speakers?