I heard on here that Purcell had some involvement with it, but when I check on imdb it doesnt mention anything nor does anywhere else on the internet. So what was Steve's input, if any, in the production of Cars?

As far as I know, Steve Purcell did story development, storyboarding, and other concept work on the film. You can see what looks like some of his art in the storyboard stills/concept work that's been released. For instance this sketch (http://www.imdb.com/gallery/ss/0317219/Ss/0317219/1.jpg?path=gallery&path_key=0317219) looks pretty Purcell (check out the Route 66 sign). I also predict we'll be seeing him mentioned on the Cars DVD special features in some capacity. I hope so at least.

I also predict we'll be seeing him mentioned on the Cars DVD special features in some capacity. I hope so at least.

He's appeared in a speaking role in at least one of the video podcasts available on ITMS for Cars, the one about the Route 66 roadie the tech crew took.