The A/C system operates when the defroster is on - this is to de-humidify the air being blown on to the windshield and prevent fogging. See the Owner's Manual.
There may be a problem with the duct system; like a vaccum solenoid is leaking or something. The heat duct door should be fully open, IIRC.
If you have the electronic Climate Control, there is a self-diagnostic program that you can run, but I don't know how to actuate it on the '99 Taurii. A real competent shop should be able to help you out.
Lastly, if you're already into spending money, go to
www.helminc.com and get the factory service manuals. Those books are 4 inches thick and are WELL worth the cost! They will cover things like starting the self-diagnostics on Climate Control.
Good luck, and keep us posted.