another factor, depending on local ordinances and guidelines; if the car is unmarked, it might not be legal in stopping you. If in doubt, put on your hazard blinkers, and drive slowly to a high visibility place or even the police station (if close). If you have a cell phone, call 911 and tell them that an unmarked car is attempting to pull you over.
Don't try that method if the car is clearly marked as the officer will not cut you any slack and will definitely put you "through the key-hole". I always cringse seeing some of the "cops" shows where an unmarked car makes a stop, and a plain clothes officer runs up to some vehicle demanding that the occupants get out. Sorry but in that situation, I'm not leaving the vehicle until i have strong proof that they are police.
Making a stop in an unmarked car, is just asking for alot of trouble, and a good lawyer would have a field day with any department that tried to push the issue.