There is one episode where the Dukes are at a racetrack, registering or something, and Rosco is with them, Bo or Luke says 'That's Rosco Coltrane in the back' and Rosco says 'That's Rosco P. Coltrane!'.

I'm not sure....Hoss  or Roger may know. They usually know more than anybody about this stuff!

I don't know but it must not have been in Hazzard since everybody in Hazzard knows Rocco and he needs no introduction. They did race in a NASCAR race in "Undercover Dukes" in season 6. I'll check it out and get back to you. 

I didn't watch both episodes of the two part show but I did run them through in fast motion and wasn't able to find a scene like that so I guess my guess is wrong I guess.

I'm pretty certain in Undercover Dukes that Rosco was never in a vehicle with the Dukes; he and Boss were together pretty much the whole episode trying to figure out what they were up to, and to get in on the inside with the crime syndicate. 

Registering a horse in The Rustlers in season 2, but I don't think the scene in question happened there. 

Any chance it was on one of the reunion movies? 

You nearly threw me with the racetrack, Kamryn. The scene you're thinking of is actually when they report to Majestic Studios in the season 7 episode "The Dukes in Hollywood". The Duke cousins are in the front of the General, with Boss and Rosco in the back. The full exchange is:

Bo: Howdy. Name's Bo Duke, this here's my cousin Daisy and my cousin Luke. We're here to help Brock Curtis out with his picture.

Guard: Okay. Who's that in back?

Luke: Unfortunately, they're with us. That's J.D. Hogg and Rosco Coltrane.

Rosco: That's Rosco P. Coltrane, and this is Flash. Say hello.

You always amaze me Hoss. You are the GO TO guy when it comes to trivia....impressive! That one sure had me stumped. Everybody in Hazzard knows Rocco so I figured they were in a different county but I never thought they could be that far away.