I have a 97 12v with 300K. When i bought the truck it was like this.....When i turn the ac or heat on it blows out of the dash vents the makes a little clunk and diverts it to either the floor or defrost vents. i have no conrtol with the ****, both the the heat and ac work good but i dont have control where it comes out, sometime i can put it on vent and it blows throught just fine, then i shut it off and back on and it comes out the floor or defrost when on the vent setting... What needs to be replaced here. thanks, Kodi

 

here is a link with lots of info,scroll down for your problem to HVAC.

http://dodgeram.org/tech/troubleshoot/Problems.htm

 

I would start at the vacuum pump and check for leaks at all of the lines and fittings especially the tee under the cowling, just above the engine.

 

Vacuum system most likely. When vacuum is lost, the vent doors "default" position is defrost, so you can always keep the windshield clear (safety). Definetly check around for vacuum leaks and if that doesn't pan out, check the vacuum actuator that moves the vent door in the dash.