A Nader bell is the "beep -- beep -- beep" heard when a large truck or other vehicle is backing up, according to the St. Petersburg Times' front-page story on Ralph Nader, which appeared October 1, 2000. (Neither Altavista nor Google picks up any uses of the term in real life, and the Times didn't say how Nader's name got on it.)
Source: Adair, Bill. "What to make of Ralph Nader."St. Petersburg Times. http://www.sptimes.com/News/100100/Worldandnation/What_to_make_of_Ralph.shtml