You must have some sort of a short or a grounding issue. I am attaching 3 wiring diagrams for the A/C and Heater circuits. Depending on your transmission and if it is a 2 or 4 door model, there may be some differences. The bottom line is to check main power lines associated with the heater, I am guessing it is the blower motor that you are turning on that stalls the truck, fllow wires that carry power and make sure that they are not exposed, brittle or have a loose connection.
Also, check all grounds from diagrams for a tight metal to metal corrosion free fit.
The diagrams are labeled 1of3, etc. And can be laid out left to right starting with 1of3 to make a complete diagram.
Images (Click to make bigger)
Saturday, February 18th, 2012 AT 1:27 AM