I have a 1998 Jeep Grand Cherokee Laredo with no heat. This is a bad situation in Minnesota in December. The fan works fine. This vehicle has the simple single temperature control (not the fancy separate driver/passenger). I can make the air even colder by twisting the control. This makes me think I am not experiencing the infamous blend door issue. One other clue is I am hearing a lot of water rushing noise. My current best guess is I need to flush the heater core. Are there any other diagnostic steps I should try?