1986 245 M46 here in sunny Oregon.
I've not been driving much lately so I sat on this for a few months: Over the winter I noticed (it was impossible not to) that after a few minutes of running the heater my vents would begin pumping white cloudy vapor out, and it could last the duration of my drive (10-30 minutes). I figured this had something to do with the rainwater leakage my car experiences, which is significant but also a very old problem that has never given me this kind of trouble before. It made defrosting the windows very difficult if not impossible (I don't have a working AC, either).

Since I am now aware of an unrevealed coolant leak on top of this, the case that the cause may lie in my heater core seems looks probable, as I've just scanned the BB forums and seen this diagnosis come from the same phenomena.

My only question is, before I tear the dash apart and everything, is if anyone has ever had this same problem of actual clouds of condensation coming out of their ventilation systems and NOT had it been the result of a coolant leak in their heater cores? Could rainwater leaking into the ventilation system also cause this type of phenomena?

As always, thanks a lot fellas