This is how Thomas and Mater figure out the joke and post credits goes in Mater, Thomas and the Ghostlight.
Mater: (eventually getting tired) Ghostlight's gonna...strip our parts and sell 'em on the computer auction Web site.
Mater: (gets up, then realizes something, and moves his hook in front of him to see the lamp) Hey, wait a minute.
Everyone else: (all laugh)
Lightning McQueen: Gotcha!
Thomas: (laughs) Very funny!
Mater: Shoot, we knowed this was a joke the whole time!
Rainbow Dash: (laughs) No you didn't!
Dash: More like you did not...
Bash: ...'cause you got scared.
Ferdinand: That's right.
Mater: No, really, we do know!
Sheriff: (to Mater) You see, son, the only thing to be scared of out here is your imagination.
Doc Hudson: Yup. That and, of course, the Screamin' Banshee. Well, good night! (quickly drives away)
Everyone else and Team Freeman: Good night! (all run and drive off in the distance, with the street lamps turning off)
Mater and Thomas: The Screamin' what? (both begin to shake again)
(The credits then show, as "I Wanna Scare Myself" plays. After the credits, a post-credits scene is shown, where the Screamin' Banshee appears behind Mater and Thomas.)
Screamin' Banshee: (growls)
Thomas: (gasps when he sees the Banshee)
Mater: (turns around and gasps, not realizing it is actually the Banshee) You better run. They say there's a Banshee out here somewhere tonight, but we ain't seen him yet. Good night. (drives away)
Thomas: Uh, yeah. What he said. (puffs after him)
Screamin' Banshee: Huh?