Sometimes, not all, when I push the horn buttons on the steering wheel, the rear wiper wipes. Any idea where the miswiring is and why it is intermittent?

Believe it or not, your problem is not all that rare. A few years ago, someone here on the BB referred to it as the "horny wiper syndrome". John is right on with the weak grounding comment. Another thing about the 740's that causes grounding trouble is that the bolts that anchor the steering column to the dash board structure become slightly loose, so that you don't have a reliable chassis ground. They are snap head bolts that when torqued in the factory the heads were made to snap off. I'm told they were used as an anti-theft measure. If you remove the knee bolster, you can access the bolts to drill them and remove them with an easy-out, then replace them with standard hex head bolts. I was warned to remove and replace one at a time so that the hardware on top of the column doesn't fall out of position. I did mine and the old bolts unscrewed quite easily.

Good luck.

I would remove, check, and re-install the horn ground.
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john

I have the same 'problem' in my 86 740. However mine is almost always on the initial attempt to use the horn. Then if I sharply tap the horn button I can get the horn to work. It's like I have to get something to move out of the way before it will work. Like a wire or something.

I'd like to hear if you get this fixed. I haven't been using the car all that much, and I think I will be graduating to a '90 740 I rescued from blown engine death.