Wiper speed is generally a function of voltage and amperage to the wiper motor, also controllable by what is usually called a pulse control board, where delay wipers are used. I don't think there's much you can do with this setup without modifying it in a way it was probably not meant to be.
If your bus is based on an incomplete truck chassis, as many are, you might look around to see if the truck model on which your bus is based had an option for delay wipers. Could possibly be swapped to what you have, but I think it might be more trouble than it is worth. Will likely have to swap wiper motors, switch and some wiring.
"Cheese Wagon" <firstname.lastname@example.org>
Former owner - 1989 Ford B700 64-pass Blue Bird (Rest In Peace, Cheese Wagon)