According to Michelle in the research department, a motorist must stop for an unmarked police vehicle if the officer engages his emergency lights and siren. According to Michelle, emergency lights and sirens constitutes appropriate identification to designate the unmarked vehicle as an authorized police vehicle. 

She went on to say, that if at any time a motorist might feel uneasy during the initiation of a traffic stop whether it is an unmarked or marked vehicle, they should turn on their emergency flashers, continue to follow the speed limit or slow down and if possible call 911 to tell dispatchers that they are aware of the emergency vehicle behind them and will pull over at the first place they feel safe such as a well lit or public area.

Hopefully this will help to answer the question.

While researching the matter I came across a fun little statement. It said, "you don't have to stop for an unmarked police car, but if you don't you will soon have a lot of marked police cars behind you.