The main character's name is Tom Weaver... same last name as Duel's main actor. A pickup truck in the garage has "GREBLIEPS PAINTING" written on it. Greblieps is Spielberg spelled backwards. Early in the movie we can see somebody who looks just like Dennis Weaver driving by in a red Valiant, the same car that Weaver drives in "Duel". The black car is very similar to Duel's truck (massive, black windows, you can't see the driver, etc.)