I believe that you have the right idea, however I will point out that I have seen marked cruisers that install 'light bars' inside the car at the top of the windshield preventing you from identifying them just from the presence of a light bar (And yes, there is a 'light bar in both the front and rear windshield; I personally would call them dash light bars [2 of them in the car], you GM may not).
So, I guess I'm saying that you would need to know what the jurisdiction your running in is using in order to know which you'll need to pull off the illusion. Heck, they may even use both kinds of cruisers to cause confusion.