I figured I would ask this question on here. i’m doing the journey in the same patch grade using the GP -38 and I'm at a point where it's raining. That's all good. It looks cool and all but. I can't figure out how to make the wpers work with out manually moving the left stick on my controller left and right. Am I missing something or is there no way to have the wipers on thier own? 
Iirc there will be some control valves somewhere around the edges of the cab (there's more than one). Follow the piping from the wipers around the cab and you'll find valves to turn.
thanks man. I’m going to keep looking I’ve tried looking everywhere and I guess they’re just something that I’m not seeing but I’m going to try your idea and see if that helps.