1999 Jeep Grand Cherokee A/C vent stuck open

jeep
cherokee
airconditioning

#1

I have a '99 Jeep Grand Cherokee with dual controls for the A/C system. After driving in 10 degree weather i noticed that the vents on the passenger side were blowing cold air. during normal city driving i do not notice this since 1. i dont sit on that side while driving and 2. i have not done any highway driving in very cold weather to catch the cold breeze. so first off i do not know when this problem may have first started. i am guessing that the vents are stuck open and the system on the passenger side will not go into recirculate. with that said, i need not tell you that heat will not flow at all on that side. how or where do i correct this? PLEASE HELP, i need heat.