4 Wheel Drive/SUVs: 2003 1500 silverado 4/4

Question
heater fan only runs on top 2 speeds

Answer
Merv,

This is usually due to a faulty fan speed control resistor.  Another possibility is the fan switch itself.

I would purchase a repair manual at your local auto parts store.  This will give you the testing procedure for each component, along with instructions on replacement.

Carl