It depends on the specific facts. Was the car running, keys in the ignition, car in gear? Was your foot on the break? Deteremining the operarting element is very fact specific but it coudl go your way depening on all the factors.
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It would seem suspicious though why they saw you getting out of the driver's side instead if getting IN the driver's side. If you have switched places then why would you be be getting out? Also - think its strange that your buddy left the scene. Werent the cops wanting to question him as well? - the person who picked hm up is a potential witness for you now...he knows what happended and would potentially need to testify in trial to say that he knows you weren't the driver.
This is interesting. You need to hire a good DWI lawyer to let the Prosecutor know that you are going to fight the "operating" issue because you would have a witness to testify that he was driving and you werent, assuming he takes care of his warrant and can testify for you in trial. You shoudln't plead guilty to something you are not guilty of.