Realtors do things like your first idea all the time. I'm always getting mail from Joe Realtor saying "I just recently sold a home like yours in your neighborhood in less than three weeks." It does let me know who the go-getting realtors are out there.
But I'd mail it. Because when people leave stuff on my door or clipped to my mailbox, that really irks me.
You might want to check with the car owner to see if you can. I wouldn't lay out the master plan to him, but ask if you can use his car in a flyer you're making.
I don't like the contest idea. Just me. Others may like it.
Postcards of the neighbor's car, but not necessarily before and after (don't want to shame him with the before pic), sounds really good.
Edited to add: Possible phrasing. "The car on the other side of this postcard isn't a show car. It's from your neighborhood. My name is Name Namely and I'm a professional detailer with a client in your area. I still have room for a few more clients. I can do a one time detailing or we can set up a schedule." Just talk to them like you were, well, talking to them. I'd also make sure to have enough of the house in the picture that they can guess who's car it is.
