[updated:LAST EDITED ON 27-Sep-02 AT 10:16 AM (EST)]Check your Gate valve out by the Fire wall. It's inline with one of the Heater hoses. Or the Dash control unit has something wrong.
It sounds like your gate valve is not opening to let the hot antifreeze in to the heater core.
[updated:LAST EDITED ON 27-Sep-02 AT 10:54 AM (EST)]Verify that the coolant level is ok as well. The temp guage may not signal an insufficient quantity of coolant in cooler weather. I would also consider a cooling system flush in the future if a stuck valve is the cause.
I had a truck that had a bad temp control ****. I pulled the temp control **** off and turned it with plyers, ah heat. Replaced **** problem solved. The old **** had a small crack that would let the **** slip on the shaft and not turn on the heat.
Being from Ohio myself, I've experianced this problem before as well. Cut a piece of cardboard big enough to cover most of the radiator. Then shove it down in front of the radiator to block the wind from hitting the radiator. Works like a champ. Of course, you only want to do this if it is real cold out.
The newer f150s don't have a valve in the heater hose to control the heat. Heat is controlled by an air damper in the heater box. The damper is controlled by a small servo motor unit on the bottom of the box and the position of the servo is controlled by the variable resistor connected to the temperature control **** on the dash. If you don't have heat, check the servo motor first. It's easy to get to and easy to replace.