The Grand Cherokee has a common problem with the blend doors breaking and this is most likely the problem that you are seeing. You can read the HVAC codes from the center panel without any special equipment and get a definitive diagnosis. Unfortunately, this is an expensive repair from the dealer. However, there is an innovative solution available that can be done by the owner at significantly less cost. Check YouTube for a video showing the diagnosis and repair of the system. “Grand Cherokee heater” will get you to the right information.