When the HVAC is completely off, I get very warm air blowing into the cabin from the defroster vents at the top of the dash.

Is that normal? In past vehicles that usually indiacted that the heater core was failing.

Does anyone have a comment or suggestion?

Thanks,

 

Coolant flows through the core constantly, so it remains hot regardless of settings. If the blend flaps are open, along with the main inlet flap, you will get hot air. Could be the inlet flap is hanging up on the linkage. If it happens only at highway speed, it could be drawing through by the slight pressure reduction created inside by the exhaust flaps in the rear of the truck. My two bits.

 

That would be my guess. It's more likely a blend door issue.

Read the RAVE section about it.