heater went out on a 91 Tempo, any ideas?

I know this is the taurus section, but I dont see one for the tempo. so thanks for sharing.

I'm fixing my sisters heater in her Ford Tempo as my Christmas present to her. Now I went to check it out today:
It blows air really well, but its never warm
When its turned on at the hot tempreture the A/C compressor comes on, and it blows cold air. There is no water in the floorboard, but the heater never worked, and it doesn't look like someone bypassed the heater core or anything.

Now, I'm thinking its the Hvac controller (head unit) that is just not sending the signal to switch from cold air to warm. Either that or its a bad relay or something like that.

So what do you guys think?

First let the engine warm up to normal operating temperature. Look to see if there is any control valve in the heater hoses. See if both heater hoses are warm, if only one is warm it is a good sign of a clogged heater core. If both are warm and no heat is coming out I would suspect either the control unit or a blend door motor. I would also try to put the control unit on vent with it set to hot. With it on vent the A?C compressor should not operate. If yur A/C control panel has the same button for defrost as heat it will operate the A/C system to dehumidify the air so it can blow dry air on the window and defrost the window