Bats,
I would use the Internet to get all the Car 54... information you can. I'll bet there's an official fan site that will have the car's make, model and year information. If not a fan site, Wikipedia or a general "cars on TV" website may have your answers. Once you nail down the basic vehicle, there's a dedicated police car modeler website that may have the decals and accessories you'll need to finish your model.
Best of luck!