I’ve seen them on a number of cars, but not all. It’s a white, *roundish thing that appears to be mounted on brackets. Possibly a video camera?
*In the same sense that the crew compartment of the Apollo Lunar Module was roundish.
Is it the lil strobe light thingy that triggers green lights?
I think you are referring to the satelite receivers used for their computers.
Is it dome-shaped? It could be their GPS system.
The police cars in my area have GPS receivers on board. The little white dome is the GPS antenna. The cars position is updated on a map back at the dispatcher’s office. If desired, they can dispatch the closest car without having to ask where everyone is. It also adds to officer safety because they can see where they are at all times. Well where the car is anyway.
It makes it more difficult for the officer to “coop”, I.E. find a place to take a nap. System could be set up to trigger an alert if the car hasn’t moved for some predetermined time.
If it looks like some sort of light, then it’s probably an OptiCom system, which is what JaxBeachBoy was referring to.
St. Urho
EMT/Firefighter
Here in Seattle, you could just park the car on a ferry boat… 
Sarge: “Johnson, where is Jones, I need to see him asap.”
Johnson: “According to the GPS info, he is driving across Elliott Bay right now Sarge.”
All the police would have to do is park under the Alaskan Way Viaduct. I drove a GPS equipped Metro Vanpool and it would not work in some places, such as through the Wilburton tunnel over I-405.
You see them on semis too.