2001 Mercury Marquis A/C / Heater unit

My A/C has a auto mode where you set the temp and it keeps it there by using both ac and heat. The other day It was very cold, my temp was set to 80. I heard what sounded like a solenoid valve or switch clicking a couple of times and the heat started when the engine warmed up. Problem, the heat kept comming out, when I turned it down it switched from floor to dash vents but was still hot, switched the system over to max ac it was still blowing hot, but the ac was working as well. All systems switch as programmed, I.E. Defroster blows out top of dash floor blows floor ac blows out vents, its just all heat all the time and the ac still comes on but is negated by the heat. My guess is some kind of door that closes off the heat but is now stuck open. Any help or suggestions