That's a story I'd love to read about. OnTrac seems to use basically junker cheaply painted white mid-90s minivans whenever I see them about. They simply slap a magnet with OnTrac on it and it becomes a fleet vehicle.

While FedEx/UPS have their own custom designed fleet, OnTrac seems to scour auto auctions for theirs.

I think for west coast customers Amazon uses them. The reason could be - http://www.ontrac.com/ontracground.asp