1998 Dodge Durango blows cold air when using heater

there could be a air lock in the cooling system and the heater core is full of air
bleed the air out or have an accredited radiator shop do it
make sure that the heater control valve is getting the signal ( cable, vacuum etc ) from the heater control switch ( climate control module) to turn the coolant system on to the heater core

are you talking about vents overhead? they only recirculate without the a/c on.

Its supposed to blow out only cold air. I wondered the same thing when I purchased my 2001 durango and then I read the owners manual and it says it is only supposed to blow cold air out in the rear of the vehicle.

check solinoid valve on heater hose for rear air

The heater core might need back flushed, I just did mine this winter. Go to dakota-durango.com and introduce your self and your rare as ever truck, a v6 durango, and have fun.

check for hot water in the pipes to the heater unit

The blower has debris in it. It need to be remove and clean out. How do you remove the dash board. See no screws.

The rear vent system has a separate heater core/blower system. heater core could be plugged up or the coolant lines going to rear heater core are plugged. could also be low coolant/faulty water pump not producing enough pressure, or air in the cooling system which will need to be bled if so.

The problem that has caused this cold air only issue is the Air Temerture Control Blend Door Actuator Motor, this motor is on the ouside of the air duct about center of the dash, in order to access and replace it u must remove the entire dash assembly.

blower motor resistor maybe why it only blows on high