Why does it overheat when heater not on

What is your question?

I was driving down the highway when all of a sudden the check gauges light came on and I saw the temp guage was at the red line so I pulled over opened the hood and noticed the radiator cap was partially open so I filled it with water put the radiator cap back on but it was still overheating so I decided to take off the thermostat drove it back home without it and did not overheat then when I installed a new thermostat it started to overheat again but when I turned on the heater it would stop from overheating. What could be the problem?

What makes this problem better or worse?

Its worse when heater is off but better when heater is on

How long has your 2006 Dodge Ram 1500 had this problem?

1 day