I have that switch in my truck with an aftermarket wiper motor setup.
It applies power to each of the terminals in sequence as you turn the speed up.
I don't know about the original truck switch, but the original Corvette switches from the 60s grounded just the "low speed" pin on low and both the "low and high speed" pins on high.
Maybe that is what it needs? As I understand it, the goal was to have power always applied so that the park function would work without having to run an extra power wire.
Let me see if I can find a wiring diagram for the original electric wipers. That should show the switch and give us an idea what it is expecting.
