On my vehicle the radio works but no sound at times. At times it happens on bumps and other times it works for twenty or thirty minutes without fail. Now it has a popping/crackling sound at times and still does it with the radio turned off. What would cause this?

 

I found the ground strap that goes around the passenger engine mount not bolted to the body but can't a online photo to show it's bolt location.

 

I think maybe it is caused by the cables etc. or is it caused by there is not enough power? Please check them first.

Btw, you can consider to upgrade the original stock radio system to an aftermarket radio music player with extra powerful features bluetooth gps navigation such as Seicane S301131 which works well for 2008-2015 Mazada 6. Good luck.