Cooling fan is only working when heater or ac is turned on, truck overheating

The engine computer looks at coolant temperature and based on what it sees sends a signal to the fan controller to spin the fan. That controller is a Pulse Width Modulation (PWM) controller, and controls the speed of the engine cooling fan.

Additionally, there is an override for Air Conditioning (A/C). Whenever the A/C is engaged the cooling fan(s) runs at high speed all the time, everytime. Not sure of the logic of two fans on your vehicle, but I doubt the one fan = engine, one fan = A/C thought. 2002 Chrysler, its possible, but I doubt it. High demand should be both fans running full speed. You should be able to tell by looking at the cables running into and out of the fan controller. Modern controllers run BOTH fans.

I'm assuming that you have used the onboard diagnostics (OBD2) system and verified that the temperature sensor is working correctly. If you haven't done that, test this first. Free at most autoparts stores. Or you can purchase a bluetooth module that mates to your mobile phone typically for less than $20. Its possible that you replace the temperature sensor, but the computer is still not getting that signal because of a cut in the wire.

One question. When the engine is off (and cold!) can you spin the blades of the fan by hand easily? Be safe, do NOT DO THAT TEST ON HOT Vehicle. Hint, use a screwdriver to push the fan blade, not your finger.

If that is whole, my guess is either dead fan or dead fan controller. Historically the fan controllers do go out. Replacement fan controllers run around $120 or so. And they are a pretty easy part to replace.

Funny I just worked on a vehicle with troublesome overheating... Spent a lot of time on it. Turned out the radiator needed a flush, bad. I really hate when folks use tap water in the radiator. That's just bad. Distilled water/approved coolant only.