It's literally under the back shelf, rear corner as close as you can get to the glass and back pillar. Pretty much inaccessible. I took the back seats out and realized exactly where "the spot" was and it would require a monumental amount of interior removal. I decided it just wasn't worth it and it helped push me in the direction of the Aragostas which could be adjusted by jacking the car up.

My RS5 is the same, need to jack the car up on each side (one point will do) and adjust from underneath. Once you figure out what you want in that department, it's pretty much set and forget.