rear door locks do not lock most of the time
89 300se. When I go to lock the doors at driver's door, the front doors lock and the trunk with no problem. The back doors do about 20% of the time. No different if I use the trunk to lock them. If I use the passenger side to lock them, most of the time, all doors/trunk will lock except the driver's back door. I have tried spraying some electrical contact cleaner down by the lock(knob) thinking it might just need cleaning. I have not noticed much change. Has anyone had this problem before? What do you suggest to fix it?