I know how you feel. My M is fine, but the front bumper on my Titan was not right...it drove me nuts. I finally took it apart and fixed it. Turns out, the previous owner "lightly" hit something, and whacked it out of alignment.
Could be the case with yours too. I would do a search for "removing rear bumper", or similar, and check it out myself.
Good luck....shouldn't be too difficult.
Tim <--- apparently likes "3" cars
2014 MB E63S
2008 E90 M3 -- Selling
2004 GT3 -- Sold
1997 M3/4 -- Sold
1995 M3 -- Sold