Adorno, in his Versuch über Wagner, states that Kaiser Wilhelm II's auto-horn honked a motif that was a "simplification of the Donner-motiv from the Ring." After a somewhat exhaustive search, I found a notation of the horn-call in the Münchner illustrierte Wochenschrift für Kunst und Leben, of 1911. I haven't tracked down any contemporaneous sources linking Wagner to the Kaiser-Hupe, which was well known in the streets of Berlin and elsewhere, but the intervals of Donner's "Heda! Hedo!" are indeed there. As Henry F. Urban's text notes, Berliners heard the motif as "Bald hier, bald da" ("Here today, gone tomorrow"). The Kaiser was somewhat infatuated by Wagner in his youth, although he never warmed to the atmosphere at Bayreuth and ultimately took more enjoyment in lighter music, not to mention cars. It may be pure coincidence that a falling major sixth and rising fourth also appear in George M. Cohan's famous WWI song "Over There." In a fine display of historical irony, Erich Wolfgang Korngold interpolated that tune into his magnificent Symphony in F-sharp, written in memory of Franklin Delano Roosevelt.
