The flags that go on windows would be easy to spot. Like the ones for football teams or put something on an antenna (if there is one) or put a light on the roof-green, small. Start a new tradition. Just make it apparent.
Lots of cars do not have antenna anymore, except for the type which is disguised inside the windshield molding. Funeral homes in our neck of the woods provide those joining the procession with magnetic roof mount funeral flags which are hard to miss. We are in a “buffer zone” between Pittsburgh and the “T”, the rural area of Pennsylvania which is still very traditional, civilized and traditional.