Heat abruptly stopped during normal operation

I could really use some help with this (I have factory air). I was cruising along on a cold day with the heater blowing out plenty of heat, then about forty minutes into my trip, I started to notice that cool, then cold air started to come out of the floor and console vents. I tried shutting off the heat, and moving the heating controls around, but no heat. Vehicle temperature range was fine (right in the middle of the temperature gauge) and I did not notice any other issues. Checks of heater hoses found both of them hot. Additionally total flush of the system occurred around a year before and the heater core, along with thermostat, were replaced at that time as well.
After some research, I noted that there were discussions around heater control valves or heater control door issues when this problem arose when everything else seemed to be working okay, but when I went to look for the related parts for the 1993 model year, I could not find anything specific to this. Does the 1993 3.0 engine have a heater control valve? If not, what regulates the heat output, a heater “blend” door mechanism? Any thoughts, insights and/or part numbers would be most appreciated.

Thanks