I have full time ambient air coming through the foot - bottom of my dash vents - and no heat or A/C or fan is turned on at all. A vacuum door issue or an electric resistor being located where ? Or perhaps a missing rubber dash grommet somewhere ?

I've moved your thread to the GMC mechanical/maintenance section.

You'll have to be more specific. Can you change the temperature of the air, the location of the air, or neither?

Well sense the heater core is mounted right there in the lower box its no surprise that i feel the same on both of my 93'* i just turn it to cool if i dont have the heat and i forget its there

Yes, I can change it to heat, floor windshield, or vent - move it anywhere. At present I have no A/C working on account of a freon leak. It seems like outside air coming in -steady too ..somehow.

Have you checked the grommet around your steering column at the firewall?

I'M not sure of this but dosent the truck have a fresh air intake just in front of the windshild? I think this is shut ON and OFF by a door inside the system

Something I seen over at the Explorer fourm where thay had a problem with the door, and was letting cold air in all the time. Their fix was to remove the door and tape shut the air in take.

You could probley do a smoke test; turn the blower on full, see if it sucks smoke into the system.

Let us know how you came out.