This is a great mod that i would recommend using , I have been looking for a mod that would let me create an unmarked police car ... and this did a good job of doing that.
1: More lights for the Roamer, preferably like the ones for the Grand Marshal that could be used for something like an unmarked version :)
2 : ETK K-Series : The lights on top look a little weird maybe try moving them inside on the top of the rear glass directly above where you put the rear lights for the Grand Marshal, or in that exact same place so it doesn't look so misplaced on the car. Another thing you could do would be adding lights on the rear bumper like you did with the Grand Marshal.
3 : Grand Marshal : add some lights to the inside on the top of the windshield like in most unmarked police cars