Hi - I found a similar problem with my S reg Mondeo in that driver's door would only lock both driver sdie doors and boot but passenger door would lock 3 + boot but not rear passenger door. Did some basic eleictrical tests on wiring to rear passenger door and concluded it was the lock mechanism itself - nipped down to my locla breakers and got a lock for £10 - rear passenger door now working fine although I still have the problem of the driver's door only only opening all but the passenger front. Remote won't open all 4 either, keeps missing the passenger front.
So, the rear passneger might jsut need a 'new' lock mechanism as for the rest, who knows??
Hope this helps a bit.