Speakers popping

[KMACD71]

I have speakers in my doors and they started making a pop noise when I power up or turn off the stereo. The sound quality fades in and out. I am trying to narrow down the causes and have started with the ground. The previous owner had the stereo ground(black wire),grounded to the fuse box screw. I tried to ground it to the wiper motor screw and can't seem to get sound from there. Am I going in the wrong direction here? Could it be speaker related instead of ground related. Any advice would be appreciated.