1990 Lexus LS400. Rear driver's side door actuator problem. Key in driver's door (and remote) unlocks all but this one. Switch on driver's inside door panel locks/unlocks all but this one. "Manual" switch on rear driver's side door unlocks/looks this door okay. Appears to be an electrical problem with the lock actuator. How do I proceed to test it and/or repair it? Does it need to be replaced? If so, what is the part I need to order? Thanks.