Hi I think I can help you with this one. I have a 1942 1.5 ton Chevy Flatbed and the controls over the drivers side windshield are to control the windshield wiper (mine has a single wiper) The left control will turn the vacuum controlled wiper on and the right control when turned clockwise or counter clockwise will adjust the speed of the wiper. Very common for these vacuum assist wipers to fail. Check your hose from the fuel pump to the wiper control for cracks. You could also have the vacuum control rebuilt as it has a diaphragm inside that also fails over time.
Good Luck
Gregg
