everthign is south park is a reference to someplace in colorado. Casa Bonita. Kenosha Pass, All sorts of shit. every epiosde. Trey and Matt went to high school at Bear Creek.
I have heard that they went to high school in Littleton and also Evergreen...so I guess that is a third location. They did go to CU and apparently lived in Sewall.
i forget which one it was but they said something off that diamond place that always has commercials on the radio, like "off the boulder turnpike at 104th" or something i dont remember but it said the same exact thing as on the commercial.
for starters South Park is an actual place in colorado and their are a bunch of CO references like in the episode where they play baseball there are like 10 different colorado towns in it.