Heater dont work

It likely has EFI (electronic fuel injection) and an idle air control valve that regulate idle speed. They can get gummed and fail to have a fast idle when cold, or get stuck the other way, and have a too fast idle all the or regularly take its time slowing down after you take your foot off the gas. An engine temp sensor can also cause a lingering high idle if it reports the engine as still being cold after warm up.

If the engine is reaching normal temp, you could have air in the system and the radiator could be low on coolant.

Jeep Grand Cherokee is notorious for broken blend-air doors. This is the door inside the A/C-heater housing that switches the air from hot to cold. You must remove the dash to repair it.

It is likely you have air in the system. Park on an incline, take the radiator cap off, start the car, let it warm up until the thermostat opens, fill the radiator until it stops going down. See if that doesn't get you some hot air. If not, try the other steps I have in my post at http://www.fixya.com/cars/r6235876-no_heat_fan_blowing_cold_air . These are generic instructions. If you get stuck or need specific instructions for your car, please get back to me.

Did you replace the thermostat? If it's bad your engine could be running cold, not letting the coolant get hot enough to get proper heat AND making your temperature gauge read cold.

either bad recirc doors or a possible bad heater core. Open the hood and when the car is warmed up to mid point on the temp gauge look on the passenger's side firewall under the hood. You will see 2 black rubber hoses running through the firewall. Feel each hose and if one of the 2 are cold then you have a bad heater core. Otherwise most likely the recirc door.

absolutley,change the heater core,its a small radiator,with a tiny hole,it still works,but it leaks,it will continue to get worse over time

If jeep is a limited model it has automatic temp control for the heater blend door, it is controled by a electric motor, and can be problematic with age,sometimes it makes a clicking noise when the blend door is stiff ,which then it must be lubed and freed up

do the heater hoses get warm?
if its warmed up and you turn the temp to cold does it get colder?
when you turn the temp to hot do you hear a pop like something is binding?
it may be just a problem with blend door or the actuator

it maybe a blend door problem. if both hoses at the firewall for the heater are close in temperature its not likely a heater core. the actuator for the blend door is located on the bottom of the heater/ac box towards the middle of the bottom. it will be a tight fit but you can remove it without removing any part of the dash. take it off and disconnect the power. start the vehicle and manually move the door and see if the temp changes