Chances are good that one, or both of the diaphrams for that actuator are shot. Check the condition fo the vacuum lines into the door first as one or both may be cracked. If the actuator is bad, Steve Brotherton has suggested replacing all three (pass front and both rears), as the added stress of a working system may blow (or suck) out the remaining old actuators.
I didn't listen, only changed the bad ones. Sure as I sit here, in 6 months I replaced the third one.
Easy to test with a hand held vacuum pump. Just be sure to isolate the system by plugging the line to the reserviour. Once that is done, you can cycle the locks in approximately 3 pumps.
'73 280SEL 4.5 (9/72)- RIP
Only 8,173 units built from 5/71 thru 11/72
'02 CLK320 Cabriolet - wifey's mid-life crisis
2012 VW Jetta Sportwagon TDI...at least its a diesel
Non illegitemae carborundum.