I'm not sure if the neon's controls are all electronic or not, but if the knob to select what vents are on moves a cable, the cable may need to be adjusted. When you try to turn it all the way to defrost and it goes back, do the floor vents still work? or just the defroster?
You can just take apart that part of the dash covering the controls, and see if you can adjust the cable or knob in anyway.
Just an idea :-/ Hope it helps.