The worst thing just before winter. My blower motor is intermitent. It works and the when I turn off the truck it shuts off. Then it randomly turns on once in awhile when im driving.

 

blower motor resistor or switch on the dash, and blower motor, when mine did this it was the blower motor I did however did try the other 2 things first.

 

First, check the ground on the firewall...