separate artistic expression (in which case there may be a copyright claim); or (2) if the organization that presented the custom car show placed restrictions on photography (in which case you would have breached the contract and the organization -- not the car owner -- would have a claim); or (3) the focus of your work was car logos and you sold these images on merchandise -- for example, selling t-shirts with the Chevrolet logo (in which case, the car company -- not the car owner -- might bring a trademark claim). Otherwise, we don't see a problem.
What about the property release? A property release is typically used to allow someone permission to enter property (typically, a house or building) and use the location in a photo or film. A property release wouldn't be required at an auto show show (unless the venue had photo restrictions).