For as long as I've owned my 2005 avant (over a year now) the driver side rear passenger door wouldn't respond to locking and unlocking requests every time. Also ocassionally the window up/down wouldn't respond either.

I suspect it may have been involved in an accident before I purchased it, so I thought some wires were possibly shorting out.

Turns out the fix was easy - the wiring in the door for the lights, speakers, window, lock, all feed to one harness which exits through a rubber boot near the hinge into the side of the car through another rubber boot.

Slightly peeling up the rubber footpad on the b-pillar (car side, not door side), clips can be pushed in which allow the wiring harness to be unhooked from the b-pillar.

My clip harness must have worked loose over time as firmly connecting it back together solved the issue.

A much easier fix than a new wiring harness or lock actuator. Got lucky.