When I switch the HVAC switch from blue to red and put on the heat, vapor comes through the defroster vents. Haven't found any leaks, but this has been going on for several months.

The entire AC system was removed several years ago.

 

are you losing any coolant to suggest a leaky heater core? Otherwise I'd think its just condensation. The AC system is used to help dry the cabin air so w/o that it would make sense that you need to get the car to warm up to dry the air out.

 

Good Point! While the coolant reservoir does seem to lose a very little bit over time, can't find any moisture on floor or under dash.