Fox News came about in 1996. I remember before that, there was just Rush and co, and they were fringe on the radio. Fox when mainstream in the early 2000s and just snowballed slowly ever since. There was never a moment where extreme polarization just suddenly appeared. Rush in the early 1990s was saying f-nut stuff back then, too.
I do think the internet was required for the final acceleration into madness.
In general commercial media is seeking ad dollars, even though its saying some crazy things it still has some limits. In addition, it has a limited number of guests and they tend to get limited to 'interesting' individuals.
The internet is not limited in such a fashion and the echo chambers and feedback loops are faster.
Limbaugh was so "fringe on the radio" that they gave him a Monday Night Football announcer spot, at least until he had a predictable response to a black football player in a position traditionally played by white guys.
