Zoltan,

Shouldn't this be on the T2 list???

Bentley says (for a '78 bus):

Wiper motor does not work:

A. Brushes worn
B. Brush spring tension too weak
C. Brushes stuck in their holders
D. Commutator dirty
E. Moving joints of linkages dry or jammed
F. Battery voltage too low
G. Armature burned by short circuits
H. Switch faulty

There are no relays in the system.  If you're getting power to the
motor and it doesn't go, you have to pull the assembly and check it
out.

The motor is removed from the frame after removing the frame from the
bus.  To remove, remove wiper arms and the nuts holding the spindles
to the body. Remove the glove box.
Remove the (i don't remember...) one or three bolts holding the frame
to the body. Remove the wires.  Remove the frame.  Be sure to note
(carefully) the location of the arm on the motor gearbox prior to
removal from the frame.  Remove and disassemble the motor.  Clean the
commutator using flint paper. (not emery!!!) Scotchbrite works well
too.  Grease the bearings, gearbox and reassemble. Bench test.
Reassemble and reinstall.

A common problem is the gears stripping in the gearbox or comeing
loose from the shaft.  If that happens, it's off to the boneyard.

Have fun.

Jim