Good news! I installed the Curt PN #56011 trailer wiring harness adapter on my '07 MS3 GT, and it works great!
Sorry I did not get any pictures (one of the few things I'm good at is sweating, and it was over 100 down in Austin yesterday)... but no need; this is truly a plug/play option, i.e. maybe a 1.5/10 difficulty.
Here is the only potential caveat: the trailer lighting I did my testing on is strictly LED. I won't be sure the converter can supply the wattage required for incandescents until I have my trailer assembled later this week or next. In the meantime, if you'll be going with LED trailer lighting and have LED tails on your M3/MS3, the Curt 56011 is a good option for you.