I recently took my 02 navigator to the shop to get the a/c serviced because the rear air was not blowing cold. After service the front and back a/c worked great for a few hours then the rear a/c started to blow warm air. The front works great and will change from hot to cold when I change temp settings. The rear will change from hot to cold also but sometimes it will start to blow warm air instead of cold air. When the rear is blowing warm air I can change the temp setting to hot and it will get hot but when I change it to the cold temp it only reaches room temp air or slightly cool air. Anyone know why only the rear will sometimes not blow cold air.