I have a 2000 cherokee xj. Sometimes the heat blows warm, sometimes it doesn't.. Also sometimes the ac blows cold, sometimes it doesn't. I was wondering if there is something like a heater/ac relay switch that could be faulty? I can't figure out what's wrong with it, it's starting to frustrate me.

 

the AC compressor has a relay, but the heat is just powered by the engine coolant.

You may have an issue with your blend door.