I noticed this some 12 or so years ago when the episode first aired, but South Park uses a snippet of "Uncontrolled Self" from Eureka seveN during the train chase scene with Britney Spears, and then immediately pivots to another song once they enter the closet. I could only find a snippet in Español, and the music is barely audible in this dub (it's very loud in the English dub). Here's the clip:
And here's the moment in Uncontrolled Self that they pulled from:
Using ViewSync, this becomes completely apparent:
Do both South Park and Eureka seveN pull inspiration from the same classical music? Is this song copyleft audio freely available? Or did South Park Studios just rip it to put it in the episode?