[Hawghauler]

It seems to me you said a "Non-electrical" smell. It sounds like you may have something on your header/exhaust manifold burning. Nothing would get hot enough to burn non-electrical components except the exhaust. As to how it got into the heater vent, It may be coming from holes in the firewall. If it's electrical, the comments stated above would certainly apply. There are also wires you may not be able to see without deep rooting that are shorting. But again not in the heater box. The most likely heat source in the heater box that could reach those temps would likely be the resistors for the motor. They may not smell electrical if they are in contact with combustible materials and are heating up. If you say the motor wasn't on, I'd consider checking the motor wiring to see if your getting power unintentionally, i.e. faulty switch, shorted wires, faulty wiring...
Good luck