I'm not really sure on the particulars but when I put a stalk from a 78 240 in to a 77 300D I had the horn was always on when I turn the key on. So there is some differences in the way the wiring is in the switch. It might be associated with the 300D having cruise and the 240 didn't. I know the cruise is a different stalk but it gets grounded through the wiper stalk.

I also noticed today when tinkering with a couple spare stalks that the internals of the wiper switch at the end of the stalk are different. They are constructed a little differently.

__________________