[Script] Any car, police car
I imagine, you enter a car, press a hot key and get a menu, with "settings" and "install police car system" after installing the system, car would have a siren now, like police cars. And in settings tab you would have an option to spawn 4 props (there are a lot of free public small unmarked police lights resources), which you could adjust using xyz axis (to fit any car you want) and those lights would boot up when activating siren.
Only problem from scripting point of view I see regarding those props glowing at decent brightness and over lets say front glass of the vehicle. Or maybe just use small light that FBI buffalo uses at the grills.
Would be very cool for FiveM server for undercover cops, would prevent meta-gaming because people wouldnt be able to recognise cops by vehicle model.