I was not driving, how can they prove I was DUI?

Under Florida law the prosecutor can prove you are guilty of DUI by showing that you were driving or in actual physical control of a motor vehicle and that your normal faculties were impaired. A person is in actual physical control of a motor vehicle when they are in a vehicle and have the ability (present and apparent) to move the vehicle.

This means that you do not have to be driving to be convicted of DUI. Actually, a lot of DUI arrests begin with the accused doing the right thing by deciding to sleep inside their car outside of the establishment in which they consumed alcohol.

There are potential legal issues in actual physical control cases that deal with parked cars such as where you were parked, if you were parked legally and whether the police had the ability to even approach you while you were in your vehicle.