I dont need a new amp. I found the amp, its placed on the trunk where the cdchanger is.

I´m asking why is the Harman Kardon subwoofer working good when standing still but soon as the car starts moving the subwoofer starts glitching and sounds really bad. Perhaps this doesnt have to do with the amp at all. Can it be some cable that is not connected correct or not connected hard enough? Or some error message that need to get removed? Nobody with these problems?