I have the factory 1998 V70 wiring diagram. All 4 lock motors are wired in parallel; polarity of the applied power determines direction of motor rotation, and therefore, lock or unlock action. Since the Lock function works OK, I think it's safe to say that all of the wiring and connectors, as well as the motor, are good.

However, according to the drawing, some models (it does not say which) include limit switches integrated into the lock assembly. The limit switch turns off power to the lock motor whan the actuator reaches the end of its stroke. This prevents the motor from overheating if a child holds down the switch for a long time. You may have a bad limit switch, or one with worn or dirty contacts. You will have to remove and disassemble the mechanism to find the switch and clean or replace it. You might be forced to replace the entire assembly if the switch is not readily serviceable.