I had originally bought my jeep in april of 09 so there really was no need to try the heater on the test drive and even the best of us sometimes forget those little details when buying a used car. It wasn't until fall of 2010 that i noticed the problem as I didn't drive it from september 09 to march 2010 ( i was medically unable to drive). When I noticed the heat wasn't working at all. i went to my local jeep dealership and they told me that there was a good chance that the blend doors (the doors under the dash that open and close for hot and cold air) were broken. In order to fix this problem they had to remove the whole dash and they did find that the blend doors were broken. After that the heat got a little bit better especially since i replaced my thermostat and coolant. But the heater still wouldn't get very hot and I wasn't even able to use my defroster in the winter. When I went back to the dealership they told me that the only other cause was that the heater core was plugged up which is apparently a common thing with the wj body style. So an extra $1000 later the heater core was replaced and from then on the heat worked beautifully. I will say that if you come across the same problem and are willing to fix it make sure you replace both the blend doors and the heater core at the same time and save yourself the extra money and hassle and you will not be disappointed. The '99-'04 grand cherokees are very good, reliable vehicles and I very rarely ever hear of one failing on anyone.