i am in the midst of doing this swap. i found a pair of perfect leather seats that had sheepskin covers out of an 85 SD. even the horsehair underneath looked like new.
i found the wiring diagram for the seats on the startek info site in their online FSM. for some reason they don't show a link to the W126 service manual, but i found it by a link posted somewhere on this forum.
the switches and the wiring loom to the motors etc underneath the seat for the most part match up with the switches i pulled out of the door of the same car, but there were extra wires and i wasn't sure how it all fit together so i had to do some hunting around.
since i don't adjust the seats much i was going to put all the wires into a loom and stash it under the seats, although the glovebox is an interesting idea...
also, i had to figure out how to move the seats in order to get at the bolts in the wrecking yard. (gotta love smartphones...) i found a post and the instructions to connect the + side of a 14.4V cordless battery to the fused side for the seats (had to hunt around to find a fusebox cover from another car) and the - from the battery to the brake cylinder. worked like a charm!