I wouldn't pull over for them until they present proper identification. If someone in a Mustang tried to pull me over, I would call 911, and ask them if they have any of these cars on the force. If so, I would ask them to read the license plate number and describe the car so I could confirm whether it really is a police officer.

Fuck Fairfax County. Their job is to protect and serve. How are they protecting and serving by pulling people over in unmarked, civilian cars? Fucking ridiculous.