In the movie, Vacation, what exactly is an optional rally fun pack?

No. It’s not to do with the CB because Clark says, in the movie, ”“I distinctly ordered the Antarctic Blue Super Sports Wagon with the CB and the optional rally fun pack” so that would indicate they were two separate options.

We never find out what the rally fun pack is because Clark ends up getting the Wagon Queen Family Truckster which presumably didn’t have the fun pack option.

Just a guess, based on how over-the-top hideous the Truckster is, I imagine the Super Sports Wagon would have been its opposite. An exceedingly cool car and the Rally fun pack would have included spotlights, custom horn sounds, superfluous chrome pipe stacks and spinners on the wheels… in short, all the things that get attention for cars at rally events.

That makes sense, thank you.