1990 240sx shotty defroster problem

[nismologist]

so two days ago my defroster was working fine. last night i was driving down the road with the heater coming out of the floor vents and i noticed that my window was fogging and go to switch it to defroster, but it stays on the floor...so i try to go to the front face vents but it doesnt switch. i can turn it off and on fine, but my defroster refuses to go on. any suggestions, previous experience with this problem, any help would be GREATLY appreciated.

thanks in advance nico