I have a similar situation on my 67 with an aftermarket wiring harness. I spent a lot of time measuring resistances for the motor and the wiper switch. I ended up using trial and error. The first time the wipers only went slow and in the fast position it didn't work at all. I believe that I had to switch several wires around, but I got it to work. You could install a ballast resistior as a temporary voltage drop to keep from damaging the new motor while you are trying to figure it out.
67 SS 427