My guess is that neither lock is malfunctioning... rather the problem is probably in one of the wires that plug into the passenger door lock mechanism. Again my guess is that the problem is caused by corrosion either at the plug or somewhere along the inactive wire.
Has the floor of your car ever been flooded... especially back seat passenger side?
If I recall correctly there is a wiring harness under the rug just about where the right back seat passenger would put their feet. The wires are brown or brown with black stripe. A number of them run together at that point and are crimpted together. They tend to corode with a green powder... especially if the rear window has been left open during rain storms.
Check the plug at the front door lock first and then if necessary do the rug thing. To access the junction of the harness... remove the right rear passenger side door sill, rear seat cushion and whatever else necessary to lift the rug. lift the pad under the rug and cut open the harness with the brown wires in it. Find the junction and repair as needed. I soldered mine.
