Sounds like you have the beginnings of an a/c vacuum leak. Our Expedition did the same thing and eventually it defaulted to blowing out of only the heater vents, which is the default when things aren't routing properly.
Whats the problem? Blows cold air with the heat settings on?
Clogged Heater core... common problem with 2002 dodge
I assume your car heater is blowing cold air, when the heater is on. It seems that, you have air on your coolant. that air pockets blocks the cooland to travel through your heater. it may be a sign of a bigger problem such as blown head gasket in the old cars, such acura legend
Your average A/C unit blows heat through all vents.
Some problem with the fan or related components most likely. the heater core has no moving parts and the fan just blows air through it.
Low coolant level, or a plugged heater core.
The heater may not be on - but the fan still blows air through the vents when they're open !
Blown Heater Core - The heater fan blows past the heater core and if it has a hole in it your antifreeze ios being blown onto your windshield.
You are in the beginning throws of blend door failure. Check www.heatertreater.net for more information on diagnosing and fixing the problem.
it is a vaccum hose that has a tear, replace hose that is most of the time by your a/c tank under the hood.
Same way the heater on your truck works... coolant is run through the engine system, becomes heated, is run through a heater coil, and a blower blows the heated air around the heater coil through the ductwork to heat up the vehicle.
The heater control valve is bad. Sometimes the dual heater control valve will go and the AC will blow full heat.