For me replacing the speakers did the trick. I put the original speakers in some speaker boxes I slapped together and lashed them up to the clarion stereo I had in the HHR, while I was figuring things out,in the garage. The HHR speakers are still playing fine there.

The replacements in the HHR are also working just fine.

 

Yeah it could just be corroding connectors. A lot of water gets in there, mine were soaked when I took them out. Who decided rubber window seals were a bad idea?