I have a copy of a service manual for my '72 that is a reproduction of the original, I also have an owners manual by the same service. The Service Manual does have various wiring diagrams. These were both reproduced by "Secretarial Services", PO box 484, 512 N Main St., Sidney OH 45365.
I'm not sure of the cost, I got mine with my trailer used. I think the PO said around $70. I don't know if that was for one or both.
If you have trouble or don't want to pay the money, e-mail me and I'll try to help. firstname.lastname@example.org