Usually the first place to start with intermittents is with the relays in the circuit.
But just for fun, with the ignition OFF, swap out fuse (actually a fusible element) #19 (30 amp, pink) under the hood with another one of the 30 amp-ers & see it anything changes. You may note that if you pull fuse 19 you will kill all power to the HVAC head unit.
If no joy, swap out the HVAC BLOWER HIGH RELAY in the cab fuse box. There is an identical relay in the engine fuse box you can swap in.
There is also a HVAC Run Relay that's internal to the BCM that would require swapping out the BCM to change. (Why they did that I don't know).
Last edited by greenman; 07-16-2011 at 12:02 PM.