I've got 3 track days planned this spring/summer as well as hoping to hit the mid-11's at the strip so this mod seems to make my wife ok with me spending some money to go even faster!!! Still have the stocker and this harness will tuck away in the back seat pocket when not needed. THis is a Schroth Rallye 4, 4-point Harness. The shoulder straps attach to the back seat-belt bolts. Ordered from a place called soloracer.com. Sorry for the crappy pic.
I had this same harness in my M3 when I raced. It's awesome, you will love it! Definitely keeps you in your seat and really easy to install. Nice purchase!