WARNING: This page is about a different car, the 2009 Audi TT and 2008 Audi TT. However, it is still accessible from the selected car via links, so may be relevant.
-- Knock rubber through at center of sealing cap using a screwdriver -B- and pry out cap in direction of -arrow-.
Fig 1: Identifying Screwdriver To Knock Through Rubber At Center Of Sealing Cap