So, you've decided to hang a metal or plastic facsimile of larger-than-human testicles from the back of your automobile.
The imagery here, for me, is always very confusing. For one thing, you're presumably trying to put forth some positive PR for your actual cajones. But the location of these swinging grapes imply that the truck (it's usually a truck) itself is the dick--in which case, your poor huevos are grotesquely undersized.
Colin pointed out that they're often attached to a trailer hitch, so perhaps that is supposed to be the phallic portion of the symbolic genitalia. If that were true, then the penis would be protruding from, essentially, the asshole of the car, which makes your twig and berries either very deformed, very gay, or both.
The message gets even more confusing when they're constructed of blue plastic, like the pair I saw this morning. If I weren't getting laid, I don't think advertising that fact would help the situation any.
The trump card, of course, is that such an excessive display of manhood is a surefire indicator that you're feeling inadequate. It's bad enough to buy a large truck because you have to feel bigger than everyone else on the road if not in the bedroom, but to add the balls just points out that your truck is a phallic symbol. Unfortunately, it also indicates that you still don't have the intelligence to figure out that you're sending a very self-depreciating message.
Colin and I have a mutual friend from college named Tommy. Whenever someone who was obviously compensating would drive by, he would yell at the top of his lungs, "SORRY ABOUT YOUR PENIS!"
Sorry, Mr. Ballsack-hanging-from-your-bumper, about your penis.