What would make the air change from blowing out the front vents to blowing from the defrost vents when you step on the gas on a 1998 dodge pick up?
i think someone has vac. lines wrong The problem is the engine loses vacuum when you accelerate. Vacuum is what operates the doors that control air flow through your ducts. I had the same problem with my 1998 Ram when it was new. They ended up putting what amounted to a one way valve in the vacuum line. I have owned four of this body series Ram trucks and they all have done the same thing brand new. Its not a big deal, but the previous answer is the best and easiest remedy that can be done. Be careful if taking it somewhere to have it installed for you, it takes a mechanic who doesn't know what they are doing only a half an hour.