I’m a geek, so the message on the read windshield of the SUV in front of me made me laugh:
You can’t tell from this photo, but it’s a rental (there’s a National sticker on the bumper). That got me wondering whether this was the most or least popular SUV to rent. Would people want the crazy back-window slogan? Do they get a discount because they’re advertising while they drive?
DarrenBarefoot.com 
Are there other companies doing this too?
Brian.
Rental car companies actually do leases, much like a car dealership does.
It’s something like a long term rental, and the companies who use a service like this usually do it because they can write off 100% of the lease cost at tax time.
That’s why they can afford to decal the vehicle. They have it for a long term.
Of course, it may also be an exchange of services for adverts. That’s possible.
Funny slogan, too. I like it.