I tend to agree with you that it could be the motor cam reed switch contacts related to the motor cam itself, based on your description of items 1 &2, and my limited experience with my rear wiper on my ‘87 – but I’m a little confused given so many guys said they solved a similar issue by replacing their stalk switch. My understanding was that as long as the wiper motor started working (assumes stalk & relay worked and power got to the motor), then the parking power would bring her home. The parking power is unswitched and should be present at the motor no matter what position the stalk switch is in, b/c it is not routed thru the stalk. It was the action of the wiper motor and its cam driven switch that removes parking power to the motor at the motor itself. The reed switches in the stalk and motor do act up however when they get old, so ymmv and hence the stalk solution. I’d recommend carefully checking for unswitched power on the applicable motor terminal but I don’t have a schematic for that year.
edit: hopefully this is a better help / correct vintage/ looks like the path could be thru the switch too:
http://forums.pelicanparts.com/porsche-911-technical-forum/605809-mystery-electrical-component-wires-74-911-a.html