Weird stereo issue

I have a 2001 Mustang Gt with a Sony cdx-f5700 head unit going into a JL 300/4 amp. In the front I have 6.5 Infinity seperates and in the rear deck I have Pioneer 6 x 8's. Every once in a while the rear passenger side 6 x 8 gets fuzzy. All other speakers sound fine. I have tried new rca cables,new speaker wire,and a new speaker. The head unit is not that old. I assume it is the head unit,but is there anything I might be missing? I also have a JL 500/1 amp going into two 10 Jl's. Sometimes the speaker is ok,but othere times as soon as I turn on the stereo,it is fuzzy. Please help.                          Jim

I'm more confused now. Stereo sounded fine,then there was a little pop,and speaker became fuzzy. Switched rca's at the head unit,and same speaker remained fuzzy. Switched rca's at the amp,and the same speaker was fuzzy. Yesterday I temporarily hooked up different rca's and different speaker wire from the amp,and it was still fuzzy. I then hooked up a different speaker,and it was still fuzzy. Now the only other weird thing is this....I had diferent amp in there before the JL. Initially,I had the speaker wires mixed up on the amp. Had first amp in,and same speaker is fuzzy,awitched amp,and screwed up wiring,and front left speaker was fuzzy,switched amp wiring back,and same rear right speaker is fuzzy. It has to be in the wiring,but what am I missing?

OK - Check to make sure the speaker wires do not have strands touching the terminals (like the + speaker wire having a strand touching the - terminal on the amp).   Check to make sure the speaker basket and connectors is not coming in contact with the metal on the car.  I assume when you did this test you used the same channel on the amp if not use only one channell at a time to test speakers, if 1 channel works fine for all 4 speakers, go to the next channel (LABEL the wires, like so  FL (front left) FR (front right), also lable the + and - on the wires.