Saw a SUV in Vegas with the typical stick figure family on the back window. Under that, in clear, well applied lettering: The Ass Family … Lard-Ass, Cheap-Ass, Dumb-Ass, Kiss-Ass.
I’ve posted about this one before:
I saw one of those awareness ribbons that simply said, “ribbon awareness.”
Re the "My Family " stickers, I’ve seen this one before and quite like it.
Two unique stickers:
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on a motorcycle: “MY OTHER RIDE IS YOUR MOM”
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on a pickup truck towing a trailer, a sticker on the trailer said “PASS WITH CAUTION - DRIVER IS MASTURBATING”
Sticker on the back of a truck:
How’s my driving?
Call 1-800-EAT-SHIT
I just saw a red SmartCar - the little 2 seater - with a sticker that was a white lowercase ‘m’ that filled each door.
Well, it’s not really a sticker, but I have an… extremely high-performance vehicle whose license plate is 4 MPG.
On a “save the greenbelts” plate.
When I met my husband, I drove a Celica convertible. It was black. The license plate holder in the rear said “All Men are Animals, but Some Make Good Pets.”
On the front, it siad “Sometimes I go Topless.” He married me anyway. 
Some years back, as a member of our local fire dept., I responded to a car crash. Small shitbox of a car being driven way too fast by one of our local delinquents had rear ended a trailer truck. Car was wedged completely under the truck body. Delinquent missed being decapitated by a matter of inches. He had one of these bumper stickers on his car! Needless to say we took lots of pictures!
Haven’t seen it firsthand, but I once saw a photograph of a DeLorean with a “Things go better with Coke” bumper sticker.
John DeLorean - Wikipedia for those of you too young to get the joke.