Home Fleet Vehicle Program
You won't have to worry about reporting any officer who uses their city issued car for personal use while off-duty because what you are describing is actually what is more commonly referred to as the department's Home Fleet Vehicle Policy but it has more purpose than that which you describe. Yes, this 'Take Home Vehicle Program' was established several years ago to promote the philosophy of community policing, and to effect a reduction of crime, and to enhance the perception of safety for all residents (including YOU) and visitors in the City of Fort Wayne.
Under certain circumstances, it is policy that even if we are off duty but in our city issued vehicle, we must respond to any crime in progress; assist a disabled vehicle with occupants; assist with a traffic hazard; respond to a vehicle accident; respond to a person/man-down, or initiate a traffic stop for a flagrant traffic violation. There are many instances, DAILY, where an off-duty officer in their city-issued/take home vehicle has responded to a bank robbery, missing child investigation, critical injury accident, unconscious party, burglary in progress, and so on. This has come to be a valuable program for the community. You should instead be writing a thank you letter rather than asking how to report it.
Maybe you should direct your frustrations at other agencies whose employees have city-issued vehicles and ask those agencies to justify their personal use of those vehicles. That's just a suggestion though, Guest5150.
As for where to report an officer's improper conduct, you can refer those complaints to the Internal Affairs Bureau at 260 427-1424. I like to reserve the discussion forum for topics of discussion and not for complaints.