So I've got a '98 Grand Cherokee and this winter the heat stopped. I'll list everything I've found to give a better idea.

• vehicle is not overheating
• engine temp is good
• replaced thermostat
• it doesn't blow freezing cold but certainly isn't heat
• if I have the fan sitting on low speed then I can feel warmth
• all fan speeds work as does the defrost, just doesn't warm up
• If I let the jeep idle for about 30 minutes it defrosts a small portion of the windshield at least

Needless to say when it hit -41°C here for a week straight it wasn't the most fun thing to drive.

 

Check your blend door. It is easy to check, should be under the dash and between the transmission hump and glove box...closer to the hump. If you move the temperature selector, it should move if it works if it doesn't move you found your problem. Unless of course your heater core is plugged up.
You can replace with a parts store one, or you can take yours apart and see if you can fix it. Its a simple thing to do and usually is frozen in place by dried up grease.

-41 temps, and I wouldn't leave the house unless I was walking into a fire. Predicted to hit a low of 14 and a high of 20 on Monday. People here responded by buying out the bread Of course, I would rather they eat then try to drive in winter conditions because plain rain is a major obstacle for them.