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?

You may have a vacuum leak!
 

Test for vacuum leak and go from there...

door locks/vacuum leak
 

Can you give me some details or directions to where I can find info on this? Where would I be testing(certain lines or just general all lines)?

Do you hear any noises whent he locks try to lock?

I had the same problem on my car, the passager rear door would not lock.

I took ta panal apart and saw that the vacuum tube was rubbing against a sharp part of the inside door. I took some shrink tubing and re-attached it and bound it with some duct tape to keep it from chaffing.