Heater was blowing cool air 2006 Dodge Stratus

2006 Dodge Stratus

couple of weeks ago, my heater was blowing cool air. when we got back in car later it worked fine. 2 days ago the car light come on while parked and i turned on the heater to help cool it down. when i got home and checked the overflow it was half full. about where it said max. i opened the radiator cap and there was no antifreeze/water to be seen. you could see the thermostat and it was dry. i am taking it to the shop Thursday. also the fluid in the overflow was a rust/brown color. should i use more antifreeze. should it be full

The cooling system will need to be pressure tested to identify any leaks found. The leaks need to be repaired. Then the coolant should be flushed and topped off with fresh coolant. After that, the system should be pressure tested again to assure there are no more leaks present. At this point the vehicles engine can be operated safely to check for proper operation. I also recommend having the thermostat replaced at the same time as it is under $10 and all the fluid is already being drained. I would not try to add or do anything to the vehicle. This will allow the mechanic the ability to properly diagnose the issue you are having without any outside deterrent.