Assuming from your post you already have a heater unit and probably only an on/off switch just put a resistor like Tim suggested ( you can find them at better FLAPS or electric motor shops) in the line between the switch and the fan motor. Be sure to mount it clear of everything even including feet and legs.
I actually took the wires through the firewall and mounted it on one of the heater mounting studs in the engine bay. Works like a charm.
