Are you sure the "frost" wasn't condensation on the outside. It's got to be miserably hot and humid in Florida this time of year. If the AC is blowing cool air on the inside of the windshield you will get condensation on the outside that the defroster will never get off. Turn the defrost off and use the wipers! Defroster mode is made for condensation on the inside when it is colder on the outside of the windshield.