This fellow made one out of an Impala sedan, they might be able to give you a few pointers.
However, according to Florida State Statuates;
It is expressly prohibited for any vehicle or equipment, except police vehicles, to show or display blue lights. However, vehicles owned, operated, or leased by the Department of Corrections or any county correctional agency may show or display blue lights when responding to emergencies.
Of course, that's just me googling. You could always contact your local police department and ask them "Hey, have you ever seen Ghostbusters? Yeah well I'm building my own Ecto-1 as a project. Am I allowed to install a blue light bar provided that I keep it covered while driving, or is it not allowed at all?"
There has to be SOME sort of caveat or something somewhere, because otherwise how would tv and movie productions be able to film scenes with prop police vehicles in the state, other than by renting them from actual police departments?
I have seen that some vehicles are permitted green or amber flashing lights, no mention of purple that you usually only see with funeral processions - so that might be your alternative.
I really have no idea what I'm doing.