I had the same problem with my '91. I found a bottle of "Anti-Fog" by 3M at auto zone for $4. just clean your windows on the inside really good and apply this. Works a lot like Rain-X for the inside and it works great.
In Flagstaff Arizona you probably don't have a lot of humidity in the air. My guess would be your heater is in constant recirculation mode--pulling "wet" air from the cabin and recirculating it. If it were pulling dry, Flagstaff air from the outside, you should probably have clean windows whether the heat was on high or not.