This picture shows what lies beneath the cover. There is a reed switch that is actuated by a nylon pin that raises when the motor turns. I couldn’t determine how the reed switch functioned, but in my case, it made no difference at all as there was a broken part that you can see in the second picture. It is made of spring steel and is formed into a ramp shape to raise the nylon pin and use it to make and break the reed switch contact as it rotates. When I removed the switch plate, the broken part of the spring-steel ramp fell out, so. at least I could now visualize how the adjustment is supposed to be made. You can also clearly see the three notches in the rotating plate so it can be moved with a flat screwdriver to change the point at which the pin engages the spring ramp.
I thought I was in luck as my parts car had the same motor, so it sould be a simple swap, but my joy quickly faded when I discovered that motor was locked up and wouldn’t move without damaging. However, I thought I could at least swap the ramp spring, but it too was damaged with a bend midway up the ramp. I tried straightening that part of the ramp, but the bend was too sharp and couldn’t be reformed properly without breaking the brittle metal.
Finally, I decided to forget trying to create a park position. As often as I might be driving in rain, I can just use the switch to manually park the wipers. Before I reassembled everything, I removed the sixty-year-old crusty grease, oiled the plate and greased the gears. At least the arms are now synchronized, and the speed is adjustable with the dash switch.