merely applying green reflective material to the side of a crown vic, mimicking tha pattern on the side of a BP vehicle, would be effective enough.
3M brand ScotchLite
http://cms.3m.com/cms/US/en/2-135/cciurFQ/view.jhtml
Check you local plastics or sign supplies store.
"Effective" enough to cause someone to believe you are a peace officer?