01 Jeep Grand Cherokee defrost no working. Blows air but cold?

There was no reason to change the thermostat. The engine wouldn't run cold with a leaky one, just not as hot as it should. Feeling both heater lines under the hood to see if they're both too hot to touch should tell you if you're getting coolant through the core.

Clogged cores are common. You can usually just take the heater lines off in the engine compartment and blow out the core with a garden hose. The blower resistor and switch just control fan speed.

They don't have anything to do with hot and cold. The worst thing is an inoperative blend door. That's when the core is hot, but the door that blends hot and cold air doesn't work.

It's a real bear to get to on that vehicle.

The most common problem on 1999-2004 JGC is broken blend doors. The blend doors switch the air between hot and cold. When the blend doors break, gravity causes them to fall down, which is the COLD position.

Google 'grand cherokee blend door' and you will see this is a VERY common problem. The blower motor resistor is a very common problem also, but this will cause NO air to come from the system.