Elec. Wiper Motor 6V.
Have you thought of "other options? Just how often do you NEED wipers in the car? Could improve the vacuum motor operation?
1--Investigate a dual diaphragm fuel pump
2--Install a vacuum reservoir
3--There used to be electric operated booster vaccum pumps
4--With today's diesel engines, maybe rig a belt operated pump off one of them?
5--Personally I'd be converting the car to 12V!!!