[90Xjay]

If the smell came thru the vents, I still would not overlook a very minor seeping leak in the heater core. This would not always leak under the vehicle. Check for dampness in the pass side floor near the fire wall.

Also when you fire a heater up the first time in for the season really good, it will stink some. You are smelling a summers worth of dust and junk that collects on the coils. If you have central heat in your home, you know that it smells when you first run the heater for the fall.
I run a foam cleaner that I inject into my airbox between the blower and heater core and it really cleans out the coils. The stuff that comes out of there can be nasty, mold, mildew, dust, etc..