Sunflower - since I feed the birds sunflower seeds, I always have a few sunflowers pop up around the garden. I love the watch the yellow finches balancing on the flower head when it goes to seed. besides tulips sunflowers come in
This is a really sweet image of sunflowers, and with the colours and farm connotations, it could be a really interesting theme for the truck. Although, this image seems a bit old fashioned for the modern theme of trucks.