Why does your heater blow one side cold and one side hot On a 1999 Jeep Grand Cherokee unlimited?

99.99% of the time this is the common blend door failure. You can go to 100% by reading the HVAC dianostic codes. With the HVAC system on, hold down the AC and re-circ buttons simultaneously while turning the drivers side temp knob one click clockwise. Codes will flash on the temp LCD display. If you see 54 and/or 56 you have found the problem. More information and pictures is available on heatertreater.net.