The next thing Jon knew he was waking up in the back seat of an old car. He felt sweaty and could smell the musk of another man. As he got his bearings he realized he was in the back seat of Biff's car, and he was lying ontop of Biff!
Biff was asleep, and naked... as was Jon... Jon started to freak out when he figured out what must have happened. Just like when he was applying the makeup, Sarah took control, he was Sarah... and now... it seems he slipped completely into her and had sex with Biff!
Jon needed to get out of there, and fast, but he couldn't move, Biff's arm was holding Jon tightly to him. Jon would have to continue to pretend to be Sarah until he could figure a way out of this... but he couldn't let her take control again.