My 1999 dodge dakota only blows out heat or A/c fom the defroster

could be the thermostat or your heater core is going

To troubleshoot this types of problem,click the link below :----http://howtobyme.blogspot.in/2012/02/heater-blows-only-from-defrost.html
--------Go through the procedure to confirm the issue.

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Engine Overheats and heater blowing cold?

http://howtobyme.blogspot.com/2011/11/engine-overheats-and-heater-blowing.html

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Heat blows cold on drivers side and hot on passenger side?

http://schematicsdiagram.blogspot.com/2011/12/heat-blows-cold-on-drivers-side-and-hot.html

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This will help.Thanks.

You may have an inoperative switch

the blend air door is stuck look down the heat vents to see if something fell in there

It is most likely a closed or mostly closed thermostat, cheap fix and very common.

PUT A HIGHER THERMASTAT IN. 190 OR 195 DEGREE

Need to check the blend door.

You may want to have the AC cleaned out.  This involves removing the evaporator and cleaning it inside out.  While you have it out, have the vents check if you have the "door" that lets the outside air in if it is working,  Just to be sure that the dust did not come from the outside.  You may also want the AC freon checked out.

could be a vacuum leak

first check blower resistor for failure and replace