How do you fix a 1992 Dodge Dakota that won't blow hot air on defrost but it will on vent?

There are a few possible reasons for this problem. Here are the usual suspects: Radiator fluid is too low. The heater core or lines are plugged. The heater controls or air flow dampers are...

You either have a bad selector switch or a bad vacuum hose at the rear of the engine.

problem with you coolant level your engine isn't being cooled properly>>> ive had my engine over heat twice in the last week first one blown head gasket this one i don't know...

Year, make and model would help but, it could be a faulty thermostat, a faulty temp blend door actuator, a plugged up heater core or the coolant system is low on fluid (leak).

Under the hood in front of the transmission dip stick is a vacuum line. If it is disconnected or split it will make the air vents not work. If that's not it check your fuses.