I never run electric wipers if the truck has been switched to 12v neg ground which many have. Instead I get an electric vacuum pump from a Mercedes diesel for around ten bucks from a pick-a-part yard. The pump is small, only requires one hose, and can be mounted anywhere. Electric wipers have 1,2, or possibly 3 speeds but the vacuum pump can have 3 or 10 or 30 or any number you choose. I send the wiper motor to Ficken and along with the pump it will run any speed and not change at all whether going downhill with a tailwind or pulling a loaded trailer uphill. I even made a foot valve that gives me the "wiper delay" option.
