I'm an auntie, not a mom or dad, but I know that the book my nephews got the most joy out of as littles was Uncle Wizzmo's New Used Car by Rodney A Greenblat. It was published in 1990, and Shelley Duvall included it in her Bedtime Stories series. I'm bummed I couldn't find a picture of the cover for you, 'cause the art is terrific. Hopefully you'll be able to find a copy in a library or used.
Uncle Wizzmo takes his niece and nephew every year on an oddyssey to Turnpike Larry's to buy his new used car. They drive through the city, past the recycling plant, watch as Turnpike Larry shows Uncle Wizzmo every kind of weird & unsuitable vehicle until they settle on one that's Just Right. Then they drive home in the new used car.
Hope you can lay hands on a copy.