The default setting is defrost on the windshield. It is either a vacuum issue, or if you have electronic climate controls, the mode door actuator is bad.
To fix, you need a vacuum gauge and strat checking vacuum under the hood, the line running into the car and inside the car. Next check all the actuators, temp, mode and recirc.
If you have electronic climate controls, you need to replace the actuator, which is found in the dash board. Search the site for blend door, ot temp door, and a write up will come up.
I wish it were the resistor, but it is not, that is for the blower fan speeds only.
