Check all associated fuses dude...
Failing anything wrong there, you may have to take your door trims off, drop your speakers out and see if theres movement in the speakers themselves while theres some music playing.
If theres no movement, check all your wiring is still intact with a continuity test on a multimeter and/or take the speaker out and run it up on a different source (lounge room stereo etc [dont overcrank it])
Just use a process of elimination until you find where the problem lies...