An argument on a petrol station forecourt led to a taxi becoming damaged, say police.
The incident occurred in Whitleigh in Plymouth on Sunday, November 25.
The witnesses, believed to be one woman and two men, were on the forecourt of the Texaco garage on Budshead Road Whitleigh between 3.30am and 3.45am.
The driver of the cab was involved in a disagreement with a fare, during which the dividing glass in the cab was broken.
It is believed one of the witnesses intervened and spoke to the taxi driver about a mobile phone.
Police would like the witnesses to contact police on 101 quoting crime reference EL/12/5179 or Crimestoppers on 0800 555 111.