A few years ago I wrote a circuit for on/off, low, high and intermittent control of the aftermarket 2 speed electric motors used in all sorts of vehicles. I meant to offer it here but then Covid and other things got in the way. I ended up having to use 2 Bosch style relays to make it work correctly. I'm going to resurrect that work when it gets cooler. I used a 6 or 7 position rotary switch with it.

~ Jon
1952 1/2 ton with 1959 235 | T5 with 3.07 rear end