I fixed my problem for less than $25. NAPA switch, crimp connectors, a diode and some shrink wrap. This fix should work for any similar wiper system.

My 64 F600 had a bad wiper switch. Good wiper motor, good wiring - bad switch. 2 speed wiper, no windshield washers. Switch can't be found and NOS is rare and expensive. Looking at the wiring diagram for my truck the original switch on HI sent power to the HI side of motor, on LO sent power to both HI and LO side of motor - odd but that is how the diagram worked. I tested this by running a jumper - sure enough supplying power to both sides of wiper motor made it run LO.

So I started by testing a cheap NAPA wiper switch - Off sent power to PARK, LO sent power to one side, HI to other side. No connection between PARK, LO or HO in any switch position. I used this to wire up my wipers on HI.

I go to thinking about this and thought a DIODE should be able to block/permit power to flow if I could get one with a high enough power rating. Sure enough easy to get and cheap - $5.
I connected the DIODE between HI and LO so power from the LO position would also feed the HI side to the motor but not from HI to LO. No danger of back feed since the HI was disconnected from all else in the LO position. Wham - have both HI and LO speed wipers. Went to the store and got some piggy back crimp connectors and shrink wrap for the diode to mitigate any inadvertent shorts from the exposed leads.

I ran this setup for 5 minutes on a dry window to test the switch and diode. Both were actually still cool to the touch.

NAPA Wiper Switch PN: TWD 7521204BX https://www.napaonline.com/en/p/TWR7521204BX
Amazon Diode: Amazon Connectors:

 

Good to know. I've got a weak air motor on my 63 c850 and will soon be robbing the electric ones off a parts truck. Fortunately the switch seems ok so far but that diagram is probably going to come in handy. Where did you find it? I've been looking for diagrams for my gauge cluster and haven't had any luck.