i have a dodge truck 99.when my headlights are on.my heater wont work

I looked over a wiring diagram and there is a ground that is common to the blower resistor and exterior lighting circuit. Its located on the left side dash. A little vague but look for a ground with at least 2 wires going to it. Possible by a fuse block on the left side of the dash. You may have to remove a cover. If that looks ok this is going to require some hands on testing by a qualified tech. Alex