1992 GMC G20 van. Power locks will not unlock doors, but will lock doors. Lock switches make no noise when attempt to use, but interior lights dim when try.

Interior light dimming is a very important clue - (intelligent observation) .. That already indicates battery power is going to the door lock actuators.. Now the door lock mechanisms are more easily locked than un-locked - I strongly suspect one or more actuator motors is weak and failing which will in turn have adverse affect on the others.. This is going to require *hands-on* testing to determine where the exact failure lies.. At least we know it s not a power supply or wiring problem. I have had some success with lubricating all of the door lock components inside the doors - that is what I would try first. Good luck!