My mtx subs pop when the passenger side seat belt is unbuckled. I can't figure it out. I asked the guy at the stereo shop hear and he said it sounded like a bad ground. I regrounded the amp directly to the chassis but it still does it. A friend suggested that maybe it is a poor body ground somewhere else on the car.
Does anybody have any idea what I should be looking for?

Check your batteries ground. They can get corroded. And make sure all your grounds have good metal to metal contact.

The ground in your amp maybe going.

how long is your ground from the amp?

its about 1 foot or so.

ok cool. 12" or smaller is reccommended, and make sure it's isolated from your other wires, and of appropriate size.