I'd be suspicious that vibration has caused a micro crack in the solder connection where the speaker wires connect to the speaker.

I'm not in possession of the wiring diagram for this car. Are you? Why would a poor ground path have any bearing? I wasn't aware that the stereo used the body of the car as a conductor to the speaker. You should have 2 wires going to each speaker. While running the stereo, flex each speaker connector tab to see if there is a bad connection there.
When you turned up the bass, it could have jarred the speaker enough mechanically to restore the connection, a sign of a continuity problem at the speaker.

If that fails, can you try substituting a speaker with the same impedance -- see if it works OK. That would pinpoint the problem to the door speaker.