Generally, the motor assembly is one of those things that have no user-serviceable parts inside and is inexpensive enough to make attempted repair pointless. If you have trouble finding a part for an oddball or collectible car, it may be worthwhile to open it up and poke around. Or maybe you’re like me and just prefer to know what went wrong.

The worm gear spins the ring gear as long as it’s powered. When the switch is turned off, power continues through the always-hot park contacts – the two fingers that rest on the copper ring – instead of directly from the wiper switch.

There’s a gap in the copper contact ring. As the ring spins, the park fingers drop into this gap, breaking the circuit and stopping the motor at the same spot every time. If you shut off your wipers and they stop immediately at some random spot on the windscreen, the park contacts are probably corroded or broken. A dousing with contact cleaner and a little touch-up with a typewriter eraser should restore normal operation. Be sure to relube the gears before buttoning up.