I don't think it's a good idea to touch another person's vehicle, even if only to put a business card under a windshield. I live by a very simple rule...don't touch my stuff and I won't touch yours. Touch my stuff without permission first and we're going to have an issue.
First, let me say I completely agree with your stand on respecting other's property, but your comment reminds me of Stripes, and all I can think of to say is, "Lighten up, Francis!"
So most the time you would "card somebody" would be at the store, gas station, work, ect. Most of these places you could wait a min or two and the owner would come out, then hand them a card. Maybe make a friend or shake a hand or two. And for the places that you would be inside for awhile, park next to them as we all do now lol and put a few cards on your Jeep. Let them take one from you, because we all check out the Jeep parked next to you anyways. So you dont mess with there Jeep and if they are interested they can call you.
FWIW, the card idea WILL work.. My local Jeep Club started in Oct 2006 with 7 folks sitting around a campfire at Superlift Offroad Park.. They had 500 4x6 post cards made up from VistaPrint for the cost of shipping.. Since then we've graduated to custom made cards (one of our members is a printer) and since then we've put over 5000 cards on the streets..
We currently have about 180 dues paying club members in the Memphis area and we had about 130 folks show up at the 5th Anniversary Ride at Superlift this past October..