2003 Pontiac Sunfire Water Resonator Refill Every Two Days Could Cause S

The purpose for the coolant reservoir is for expansion, when at operating temperature the water expands and have some go into the reservoir, When the engine cools coolant /water condenses and draws the fluid back in, in you case you are loosing it, this is mainly due to a leak ,means you need a pressure test to find the leak .This can be at a number of places, the waterpump, radiator , heatercore , cooling hoses

As he said, get a pressure tester from the local parts store and when the engine is cold, pressure test the system. Once it is pressurized, then you need to look for leaks.