Check your owners manual, there's a procedure to reset it. If you push the button it should go to the vent position. Turn the knob left and it goes all the way down. Turn the knob right and it goes all the way up. I might have them backwards though....
1: Start with the ignition key in the OFF position
2: turn the ignition Key to the ON position
3: Press the rear glass control switch vent button 6 times
4: Turn the ignition key to the OFF position
5: Press the rear glass control switch vent button 6 more times
6: turn the ignition to the ON position. at this point the rear window will move up and down approximatley 20mm (0.8 in). this movement signifies that the rear window glass motor has entered into the initialization mode
7: NOTE: the rear window glass control switch must be held in the UP or DOWN position until the window glass stops moving. failure to hold the switch will end the initialization operation. then the initialization will have to be re-started.
Hold the rear window glass control switch in the full UP position until the rear window glass stops in the full up posistion
this is taken right from the Ford Sport Trac Shop Manual for a 2001
Page 501-11-111
This needs to be done anytime the motor or switch is disconnected.
here are the pages from the manual for checking the rear window
