This book is very, "American," and was current for the times in 2005, but a pretty good read on the subject if you are interested. It's not officially affiliated with South Park but uses some of its episodes as references.
Synopsis:
According to author and political journalist Anderson, for the better part of 30 years, liberal bias has dominated mainstream media. Now he reveals that the era of liberal dominance is going "the way of the dodo bird.