Yes, if you're just going to see the show and vendors, there are areas that the parking can get muddy. The secret is to be there EARLY! That gets you parking up by the gate which is nice and high and dry. I usually get there at 8:30 so I get really good parking! The "back lot" referred to is the Mulberry Street parking. It's a small area compared to the main lot but can sometimes be a good option.
I've gone there for 20 years and have never suffered a door ding. When you park, you determine how far you are going to be from the car next to you and the next car follows suit. The early people, at least, all leave plenty of room between cars.
If you're bringing a vintage or modded Mustang, enter your car into the show. Modified Mustangs are on the show field on Saturday, along with some vintage cars, and vintage Mustangs are on the field on Sunday. Unfortunately, there are no classes for Unmodded late model cars, it is a hot rod and vintage car show, after all.
One of the car clubs that I belong to, the Red Hook Car Club, does run the show along with 5 other clubs. Unfortunately, the Orange County Club does the parking so, I don't have any pull there to work out something for parking on the grounds. As I said, getting there early is the key if you're parking in the parking field.