I do not know the 124 but if it is like the 126, you will find a junction box under the carpet in front of the passenger front seat. There the hard plastic tubing goes to all the doors, trunk, and gas door. Hook up the mityvac there and trace your leak. Chances are that the actuator in the rear door is bad. New ones are not expensive and it is an easy replacement. Just be careful removing the door card and do not break off the plastic keepers; lift it up and then out to remove.