Do I need a thermostat in a 2003? I had them removed before in other vehicles and not replaced just didn't have heater or defroster - I live in Florida.

Yes. Without a thermostat your engine won't get to the operating temperature the computer is looking for so it will think the engine is cold and enrich the fuel mixture to help warm the engine. That's like driving with the choke on all the time, the engine won't run right, you'll pollute the air, & your gas mileage will suck. Newer engines run hotter & that added heat helps burn the fuel more completely, reducing emissions. Most cars these days run a 195 degree t-stat & some engines run as hot as 220 -230 degrees. That's because every pound of pressure the cooling system builds increases the boiling temp of water by 3 degrees. A 15 psi system increases the boiling temp by 45 degrees to 257.