I think window signs telling about the car are a great idea. I know that when I come across what strikes me as being an interesting car, I'd like to be able to read and educate myself about it, but that appears to be a concept that few people understand.
My 1985 Bayliff Packard combination is doubly "cursed" in that regard. People both have no idea what an 80s-era Packard is, and then have no idea what a combination is. And since only two were ever built, it literally is a car that most people have never seen before, and had no idea even existed.
So I have a window sign that I place in one of the rear side windows at shows that gives three brief histories, one on the Bayliff company and the 80s-era Packards, one on combination coaches, and one on my car in particular. You would not believe the number of people I see who put their hands around their eyes to shield glare off the window, and then look all around the sign to see the interior, and then come over to me and say "so tell me about this car". I also have to love the people who ask about the car, listen to my story, and then feel the need to argue with me about it. Um, if you're so smart about a car you've never seen before, why did you have to ask about it? One particularly vivid memory I have took place at a cruise night, when a gentleman asked about the car, and then asked more and more, and ended up spending about ten minutes with me going around the car with him, pointing out and explaining various things, only for him to suddenly turn rather gruff and exclaim "you can't bull**** me, I know a customized Pontiac when I see one" and stomped off.
I still very much enjoy attending car shows and displaying my cars, but I feel like I'm increasingly enjoying walking around the show to see the other cars, rather than hanging around mine and hearing all these dumb*** people.