I disagree that there is nothing wrong with the good guy/ bad guy method when their ultimate aims are divergent. First of all you need to clearly establish roles and responsibilities. Who runs these field crews, you or her? They cannot serve two masters. One of you needs to retreat and allow the other to do their job. There must be consistant management. Secondly you need to have a written policy for reporting and dealing with auto accidents, injuries, and damage to company property. The employees must understand that accidents will happen, but you must be promptly informed of them so that any fallout from them (property damage, bodily injury, or unsafe mechanical conditions) can be addressed. The last thing you need is a call from the police or a lawyer. Tell the guys this is the reason you are asking who id it, not to yell at them over spilled milk.

Good luck, bro.