What is the name of the 1980's movie/comedy were an advertising agent is institutionalised and the other patients intercept the advertising companies mail and do the advertising for their new friend. The advirtising company thinks they are brilliant,and the man does not find out about the advertising until he gets released and watches some of the advertising and it's the patients disorders.

I have no idea but my mum always talked about a movie that sounds like this... I think one of the ads was for a car and the slogan was "it's boxy"

Good luck finding an answer

It's Crazy People (1990)