I wouldn't recommend using an extension to insert something as basic as a Google Analytics Snippet. Each extension slows down the system and makes it less maintainable.
I don't know what your customVar does, but you might try something like
temp.analytics = COA
temp.analytics {
10 = TEXT
10.value (
first part of GA snippet
)
20 = TEXT
20.data = TSFE:fe_user|user|username
30 = TEXT
30.value (
rest of GA Snippet
)
}
Then insert your snippet where you want, e.g. headerData.99 < temp.analytics
and you're done.
Or even try with a wrap and insertData=1.
As Artur says, if data doesn't render in .wrap, always try stdWrap.wrap - it tends to add functionality (although afaik there have been efforts to pass every wrap through stdWrap by default, but I don't think it's fully implemented).