The assailant used the bar to smash the windscreen and three windows of the yellow Peugeot, which was parked on Main Street in Garton-on-the-Wolds.
The 43-year-old victim inside was also assaulted during the attack and left with small cuts to his forehead and injuries to his arm.
Police believe the incident, at about 4.30pm on Sunday, could be linked to an altercation between the two men on Eastgate North in Driffield about an hour earlier.
The suspect has been described as white, about 21 or 22, about 5ft 11 and slim, with short ginger hair and wearing a grey tracksuit.
It is thought he was riding a twist-and-go-type scooter.
Witnesses can call Humberside Police on 101, quoting reference 16/97891/18 or 16/97888/18.