I was the last car of a procession once.
The cops ‘trade off’ as the procession goes through each intersection, so the cop at the tail end is constantly changing...usually the guy who had traffic stopped at the previous intersection, who has to catch up.
We could tell that one of the cops was very suspicious of us, and pulled up alongside us, trying to peer in and see whether or not we wearing suits. He eventually satisfied himself and fell back behind us...but I can see how the last car can be hard to identify.
I’ve also been in processions where a magnetic sign is placed on the last car.
Well they apparently had some ribbon, so spotting them shouldn’t have been difficult. But that’s assuming they’re keeping up. If I saw somebody with a ribbon but no other ribboned cars within sight then I wouldn’t just let them run lights, they had become separated from the procession and were back to having to follow normal traffic rules.