Defroster on 88 Z24 doesn't work. Please help!

Of course this had to happen now that the cold weather has set in. My defroster for my 88z24 will not work. All other controls and the blower fan work perfectly. When I set the switch to defrost, all the air comes out from the floor vents where the normal heat is supposed to and only a small amount of air trickles out the defroster vents. I think that the device that directs the air flow is having problems. What is strange though, is that it has no problem with switching between the center dash vents and the lower floor vents. It just won't switch to the defroster vents. Anyone ever had this problem? What causes it and how can it be fixed?

I have this exact same problem with my 86 cavy. I have no idea what it is though, but you're right, of course it has to happen right when it starts to get cold. I see your message is fairly old though, did you find out what was wrong?