Bob,

Fortunately, that socket with 24 screws is rather easy accessible.
You'll find it when you take of the larg black plastic upholstery at the inside rear of the roof (it's fixed with some screws so not difficult). Also, you'll have to loose up a bit the ceiling. When there, you'll see how.
Than, on the inside of the left hinge - it will be at your right hand seated with your back to the front of the car - and close to the damper, you'll see the socket. It's rather big, black and banana-shaped and looks in fact very "old fashion" with his screws. But in reality it's very practical to work on.
Than, still being seaten in the back, take of the black plastic upholstery of the upside interior of the rear door (will be close to you as the opened hatch is upside-down!) fixed with some screws too.
Loosening the 12 screws connecting the rear door cable harness to the socket (mark the wires if that makes you feel more confident) will allow you to pull back the cable. As we can expect the damage in the area where the harness (rather stiff with his 12 wires!) has to flex all the time, check that out first. Don't allways expect to see some damage. Check wire after wire by feeling the insulation.
You will be surprised how easy a broken wire can be felt through the insulation.
I don't use splices to repair : I don't like them and they will make the harness
even more stiffer and thikker, exactly what we don't need. In fact the hardest part of this repair is putting the cable back through that rubber "harmonium" and fiddling it back in his place close to the LH hinge. I am not telling you a secret if I tell you I putt some vaseline to the wire harness?
I cut out the broken part and solder a new piece of wire (of the same thickness offcourse) in between. The lengh is choosen that way that my harness is about 2 inches longer than original. That way there will be more "liberty" to flex.
Last but not least think of this : as everything is accessible, verify also the situation in the RH rubber. You'll find ther the wiring for the rear window heating and more important, a clear plastic tube (with nylon tissue in it) often hardened and cracked and leaking or pinched, for the washing fluid.
I put a black and soft rubber tube in place. Haha, when you pull the old tube out be shure to have a wire fixed to one end. Guess why ( and how I found out )...
And at last... what about replacing the dampers the same time?
I didn't and still regret so that's on the program this summer as well as to take out left & right third window to put a new seal in. All the W124 wagons I know of, are leaking there...
Well Bob, I hope this will help you out. Take it easy : the nicest thing about this repair is that you can take your time because you can drive the car anytime with the unfinished repair!
If you have an EPC you can see that clamp in group 54 - electrical equipment and instruments - figure 570 - electrical parts used for rear panel door item 33. The partnumber of the cable harness is A 124 540 37 07 if you prefer to replace it.
Good luck and share you experience!

Danny