How can a repo man repossess a car that is locked in a residential garage?

NO, NO, and also NO. NOT LEGALLY. the UCC of EVERY state and the FED UCC prohibits breaking in to LOCKED DOORS, GATES, GARAGES,ect. To do so is called Breaking and Entering. To involve the police is prohibited also. That makes it NOT a civil process as a'self-help" repo is supposed to be. Call a LOCAL attorney for state specific advice.