Open the hood of the car. As you face the engine compartment, directly in front of you is the radiator. Just to the left should be a plastic bottle that resembles the picture below. This is where you add coolant. Never add coolant to the radiator directly unless the engine is COLD. The radiator cap is under considerable pressure and can cause injury. I prefer to add coolant with the engine running. Just add enough to the line on the reservoir to Hot or Cold depending on the warmth of the engine. If you're unsure , wait until the engine is cold (a few hours) and just add to the Cold line prior to starting the car.
1999 Jeep Grand Cherokee Coolant Reservoir
Hi Keep a close watch on your coolant .Also it sounds
like the sensor is failing or you have a loose wire or short in the wires to sensor.
Actually the light is on because the coolant level is low. The sensor is
located inside the reservoir tank on the passenger side near the firewall. Fill
the reservoir tank to the appropriate level ("HOT" or
"COLD") with the proper 50/50 antifreeze mix. This should shut off the light. It may come
on again after a short period of operation and cool down cycle if the level inside
the engine is severely low. I am sure your problem will be solved. Please get back to me if you have further
query. Thanks
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You need to add few ounces of coolant to the reservoir. The reservoir is located under the hood. It is a white plastic that has liquid in it. Please see picture below of the coolant reservoir. 
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