Why would a car overheat when the defrost is on?
The defrost selection cycles the AC pump, regardless of what temperature you are choosing to defrost at. This cycles the freon, and cools the evaporator (inside dash), and heats up the condenser (in front of radiator) I would check for 1. the fan working in front of the motor... 2. road debris, bugs, cotton from weeds plugging the space between the radiator and the condenser (common)
Turning the heat on would actually lower the engine temperature. It's a well known trick if your car starts to overheat. Having the A/C on causes a power loss to operate the compressor, which would also cause engine temperature to go up due to the hot air from the condensor going through the radiator.
You are not giving enough information to get a reasonable answer. It's like asking why it won't start. Any, and I mean any, detail would help. Are we talking about the engine cooling system? What are the symptoms that bring you to the conclusion it is overheating? Have you checked the coolant level? What year and size engine? Does it overheat only when stressing the engine, like climbing hills? Does it overheat only in traffic…