What to do.........

Nobody knows how to actually write a letter anymore... and notes on windshields or doors generally just get tossed. Pick your time to approach the guy- just picture how YOU would want to be approached. NOT in the middle of the work day, NOT at dinnertime. After work hours, but not just as the guy's pulling into the driveway after a long frustrating day. Then phrase it politely, like you're asking a favor- "Hey, I was driving by, and noticed you're a Mopar guy. I've got a '70 Duster (or whatever) that I'm working on, and thought you may know of someone or some place that may have *** parts I'm trying to find". It breaks the ice, doesn't put him (or you) in a spot you're not comfortable with, and opens the door for the conversation to develop in a hundred different directions with no pressure on anybody.
He could counter with "Yeah, I might know a few people" or "How would you like a whole car?" Either way, you win.
It's just the art of tactful conversation- too bad social media has all but killed it.