I can't get the back door to open using either the inside or outside handle. I have a central locking system so the door locks and unlocks with the others, it's something in the handle (someone told me that a pin probably came loose or apart on the door handle lift). When the front door is open I look back at the door fra,e toward the back door I see that there are only four bolts holding the rear door (if I'm looking at the thing right) and a little plastic thingy between bolts 2 and 3. My real concern is the wiring harness. I have a 86 and I went to the junkyard to look at a similar car (experimenting), but it was a 90-91 and the wiring harness just snapped together there at the bolts on the outside, but mine has rubber boots around the wire at both ends (frame and door) and I'd be afraid to pull on it and expect there to be a snap and there not be. No manuals, no money, no previous experience. How do you access the wire harness for the door and is it a snap inside or (god forbid!) I have to cut the wires in order to have enough leverage to get the door off so I can have someone fix it so it will open

There is a very useful entry in the 700-900 FAQ that I had to use just two weeks ago on my daughters 87 745. It details how to get the inside door panel off with the door closed (unlockable). Her car behaved exactly as your car's report. After you take off the speaker grille the screw(s) referred to in the procedure (just one on my daughter's car) is not directly visible. The head of the phillips screw is countersunk below the surface of the door panel. That screw is in the vicinity of the holes for the speaker grille mounting. Use the other door (the one that's still working) to figure out how the various clips are removed (3 on bottom of door panel + one under the red lens on the edge of the door that indicates the door is open). What I found on her car was the actuating rod that goes to the lock had popped out of one of the black plastic clips that keep the rod from flexing inward. This is a long rod - maybe 24" long. While you are in there use some spray lube and hit all the pivot points and other deserving friction areas and you'll have a door that works like new. Any more questions either post or email me directly. Cost to fix: some spray lube. No wire cutting.
Don
97 965 133k
91 245 203k
87 745 133k
82 245 306k one owner

Even if you take the hinges off, the door lock won't release so any worry about the wiring is pointless.
If it was my car I would take the cover off the good door and learn how the mechanism works and then devise a way of releasing it thru the window slot using a wire hook or whatever it takes.

It has central locking system. When I lock the doors, the door lock on that stuck door engages and when I unlock the system the lock on that door disengages. I do not think it is the lock that is stuck, it is the handle mechanism that is not connected or has broken. When I try to open the door from either the inside or the outside you feel none of the "click" that you feel on the other doors when the handle mechanism gets to the point where it is to release the door to be opened. If I could just get this door opened . . .

Contact your local car thief. He should be able to get the door open in about a minute.
Seriously though, tow truck drivers and locksmiths are experienced in getting doors open w/o keys.

It's not the tailgate door, it's the door behind the front seats. The door handle does not operate from either inside or the outside. If I could get it open, I might be able to find out how to repair it.

The main service ignition key (not the valet key) will unlock the rear hatch manually. If you can't turn the lock with your key, then you have the valet key and need to visit the local dealer for the main key. Take your VIN so they can match it. In any event, it appears to be a stuck latch and not the lock, so work on that. You can read the 700/900 FAQ under "features" button above for more insights on how to work back there.

I'm sorry Ringlee, I wasnot clear at all. I didn't mean the tailgate door, I meant the door right behind the front passenger seat. That door does not have a key, it's just the handle.