The wipers are a switched ground system not a switched hot...
You should be able to ground the HI or LO wire and turn the motor on.
Have you tried bypassing the delay/intermittent-wiper module on the outside of the steering column? It should be right behind the steering column access panel in the knee bar sheet metal.
You should be able to unplug the module and plug the switch connector directly into the cab harness. The switch pigtail will have three connections that don't have a mate on the cab harness. These are from the delay potentiometer in the stalk switch to control the delay timer. However... HI, LO, and OFF should work without the delay module if the switch is OK.