Probably the air blend door has broken, which is a plastic flap inside the heater unit under the dash, and is a common fault.
I had the same problem on the drivers side of my 2001 V8 Limited. I fixed mine with a bit of enginuity after searching on some American Jeep sites, or there is a repair kit available, search for 'Blen Dor' dual control repair kit.
the kit involves some butchery under the dash, but is better than the £1000 a dealer would want to fix it!
Ian, December 2009
