Thanks guys.
It's both left and right door speakers, yes. And nope, no speakers in the back. Standard speakers I believe, so I don't think anyone's been playing with the wiring.
So a replacement of the door speakers could be a good shout... That wouldn't be a bad upgrade in any case. I may give that a go and hope that it fixes the problem. If not, I'll go digging into the wiring, or consider a new head unit.
Cheers