My Ford Taurus won't defrost, heat comes out air vents and warm air comes out defrost vents, just won't defrost. Already fixed vacuum line from brake booster. Any suggestions on what to check?

First you need what year it is, then you need what heating and a/c system it has, but you may not be actuating the defrost mode door in the heater case. Now you need to know if it has vacuum or electric actuators. You may want to check for service bulletins. You can call your VIN into the service advisor for Ford at your dealer and ask for a check of the bulletins and messages. Describe and identify the controls to get the system identified.

Ford systems revert to defrost when they lose vacuum supply, historically.

There's some unidentified discussion in other taurus 2000 and up questions on this site. They seem to try to be discussing a similar lack of defrost. Theirs is vacuum operated in that case. see if any of their banter speaks about your vehicle information, perhaps.