My '99 diesel F250 has an electric vacuum pump to run the heater controls. I don't know if it would have enough volume to run a wiper motor, but they're cheap (although 12V). A cruise thru a wrecking yard would make them even cheaper. There's a small plastic vacuum reservoir as well. The earlier Ford Diesels ('97 & earlier) used a belt driven vacuum pump that powered the HVAC controls AND the brake assist, which would have plenty of volume for a vacuum wiper, but it would be a bugger to mount in an old truck.
