Posted by ESC on February 14, 2000
An inquiry about this phrase was posted a while back. I just found this. Better late than never.
From "Slang: the Authoritative Topic-by-Topic Dictionary of American Lingoes from All Walks of Life" by Paul Dickson (Pocket Books, New York, 1990), under "Automotive Slang," "throw under a bus -- Sales talk for selling someone a car or van with all the extras and options at full sticker price or better."