It introduces a question.What function does "So" play there?
Adding 'is' would make the sentence ungrammatical. Consider these like forms:Is there a word missing? which is "IS"?
So what is the 81-year-old Paul Newman doing IS playing
What are they doing watching TV? They should be studying!
What are you doing with my wallet? Are you trying to steal it?
What are you doing to the carpet? I just washed it. Take your shoes off!
You're right.[Are] there any words missing between "cars" and "a cartoon". I think it is "which is"