Jon, I grounded mine directly from the battery. I had two additional pigtails off the main (-) cable. One went inside the cab to a common grounding block the other to the fan motor casing. I drilled a small hole in the casing as there is plenty of clearance inside and used a self tapping screw through a wire connector. Then sealed the connection with clear all-weather goo.
I thought of doing the self tapping screw thing so now I will try it. Just the reply's I was looking for, thanks guys. The motor goes on and off if I mess with the heater box so I am pretty sure a good ground to the firewall and I will be set...
Jon - 1985 CJ7, 258, 77k, T-176, Dana 300, 1" body lift, 31's, wrangler tub, yellow!
Assuming the waggie's two wire job is a hot and a ground you could use a CJ's three speed switch and it's associated dual coil resistor which resides in the CJ's plenum to keep it cool.
The switch and the resistor will do the speed control to a single wire to the fan motor ... in theory ... [img]/wwwthreads_images/icons/smile.gif[/img]
Oh, and I grounded my heater motor by running a 10 ga black coated wire from the bottom most heater housing through firewall stud on the outside of the firewall (including bare metal circle and star washer) to a ground buss (to batt) I'd installed (ala CJdave) on top of my fender well.