The riff: Called out in an old-timey radio voice by Mike as a shot of an old, black car is shown.
The explanation: I believe this is a general joke about the prevalence of the “death car”-type attraction that one might see at numerous county fairs. The major example is “Bonny and Clyde’s death car,” the historic car the two was gunned down in. The bullet-riddled car toured the country as a tourist attraction, prompting many imitators and fakes. It really is the perfect type of county fair hokum, in the tradition of P.T. Barnum’s fake historical curiosities. The “Hitler death car” concept seems to have taken on a life of its own beyond this riff–I remember a scene in The Simpsons where Bart wrecks the supposed Hitler death car, and you can find artistic interpretations online as well.
Novelty factor: Between The Simpsons episode and general knowledge of Bonny and Clyde, I got what they were going for. Weirdly, I found The Simpsons clip in German. Enjoy.