Fred,
I just checked the wiring diagrams in my 2001 Dodge shop manual, and it looks like the mirror heater switch receives its power from the same circuit as the heater blower motor and a few other items. Obviously you would have the heater blower motor circuit (what truck doesn't come with a heater?). This circuit is protected by a 10A fuse. So it appears you're good to go there.
The wiring diagrams also show a relay for the mirror heater switch, but the good news is it's built into the dash control unit, so if you buy one of those, you shouldn't need anything else.
Good luck,
John L.