I have a 99 Grand Cherokee with the inline 6. It doesn't seem to matter what setting I put the temperature (dual zone) controls on, only hot air comes out of the vents. Even if I put on the A/C it's still only hot air. Is this a stuck vent somewhere or is it electrical?

I am not sure if this works on WJ's, but try this to extract the codes.

turn key to the ON position, push and hold the A/C and RECIRC buttons, while turning the tempature dial one click clockwise.