I've got a screengrab of a Mid-1950's Buick from a Kim Possible fansite. It's a link, but you can click on it and see it for yourself if you like:

http://www.maj.com/gallery/binomial/KimPossible/CarTrouble/Act1/pdvd_020.jpg


These are from episode #2.5(Car Trouble).





Enjoy.

Hey, more Kim Possible screengrabs:

School buses that I think are partially modeled after real ones from the 1940's and modernized. This one is in episode 1.2:




Chevrolet of some sort, from episode 1.15. Possibly an Astro:



This one from episode 2.27 looks like a Peterbilt 377. Anyone agree?



I'll find more, hopefully.

Here's another one from episode 4.7. They're clearly Ford F-150's from 1997-2003.
http://caps.kpfanworld.com/veiw_pic.php?av=901-1000&ep=grande_size_me&pic=0953.jpg

http://caps.kpfanworld.com/veiw_pic.php?av=901-1000&ep=grande_size_me&pic=0966.jpg

Anybody willing to I.D. this motorcycle from episode 2.12?
http://upload.wikimedia.org/wikipedia/en/f/f5/Exchange.jpg

A fifth message on Kim Possible. From episode 4.12, here's what looks like either a 1947-53 Chevrolet, or a 1948-50 Dodge:

A little tip; Kim Possible ran new episodes from 2002-2007, not 2002-2008.