Vehicle body wiring is in the Body Service Manual and that's a big and expensive set. Maybe a large Ford dealer would have one and a box of donuts might get you a look.
Helm, Inc is the source for most published auto service manuals - they do Fords. See Helm Service Manuals
for the E-van series of manuals.
Have you removed the mirror head and verified you getting power to the mirrors themselves when you push the switches?