I'm not sure if this is exactly the same condition as you currently have, but the "latch handle" on my '89 740 wagon rear door was binding and causing a lot of difficulty in opening and closing the door. This is the latch handle that you pull on with your hand to open the door -just above the license plate.
I was having to "push" forward on the handle in order to unlock, open and close the door.

The key was very difficult to turn to lock and unlock the door. Also, the automatic lock on this door would not engage or disengage the lock when the other doors were locked and unlocked. All of these problems were caused by the latch handle spring being badly corroded. Everything was fine with the electric lock motor, the lock cylinder, the latch at the bottom of the door and the linkages between the lock motor, the handle and the latch.

I removed the handle assembly and used PB Blaster and a gel rust remover in order to get the handle to move freely. I also removed the broken plastic bushing that was bewteen the spring and the shaft on the handle. I then used a liberal amount of white lithium grease to lubricate and protect these parts from further rusting. I put it back together and everything works like new, including the lock cylinder and automatic locking mechanism.

The handle assembly comes off pretty easy, after removing the plastic trim on the door, there are four bolts that secure the handle to the door. (Two of these are nuts and two are bolts to be removed.) Then carefully pull out the two linkage shafts that connect to the lock cylinder and one shaft that connects to the handle. There are small plastic fasteners that hold these shafts in place. It's not too difficult to do. Good Luck.