Defrost usually involves hot air from the heater core being passed over the Air Conditioning coils to dry the air before it is blown onto the windshield.
A few more details from you would help me help you. Are you saying that nothing happens in Defrost mode? Are you saying that air blows out but the windshield doesn't defrost? Are you saying the Jeep explodes when you select defrost?
Basically, if your heater works but the defrost doesn't, look for something going wrong in the A/C system. Unless the problem is that no air at all is blowing out, in that case look for a bad climate control module, bad blower motor, etc..
-Nick :!: