I'm almost absolutely sure that the ground to the steering wheel is faulty.

The first thing I would do a continuity test between the middle of the steering wheel (under the pad) and your favorite chassis ground. I prefer the outer ring of the cigarette lighter. Remember to put the steering wheel in the streight ahead position. While you're at it, check for continuity between the metal on the outside of the ignition key and the inside of the steering wheel, and the ignition key and your chassis ground.

I'm not sure where the steering column is grounded from, but I would give the universal joint where the steering wheel bends under the hood a good blast of WD40 and spin the steering wheel all the way side to side for a few turns to see if that helps.

If it dosn't, I would get a Haynes manual for the 1982-1988 740/760 and follow the instructions for removing the steering column. The ground is likely part of the bearing that supports the column.

(Other brickers - If I'm leading this gentleman into the dark abyss of confusion, please correct me.)

I hope this helps you somehow. I had typed a long shpeil about removing fuse 15 and turning the front wipers on, but after beating my head on the horn actually *failing* with it's super-simplicity, as well as it all being related to the steering wheel position, the only think that makes any sense is the column ground going south.
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89 764 GLE 150k/ 86 745 GLE 240k