Do KNOW that Schroth approves these harnesses in conjunction with a particular seat. To my knowledge, Schroth has not produced a harness that is tested and DOT approved for use with the E9X M3 OEM seats. Do this at your own risk.
My thinking is that I'd rather that the risk using a seat that is not fundamentally different from the E46 this harness was designed for. That's a trade off that I will make at this point vrs driving with the OEM lap belts which just don't cut it. Even if you pretension the belt, my mind would focus on bracing myself vs concentrating on keeping the pavement under the car.
1. Take out the back seat. If you need directions on how to do this, just close internet explorer and take it to a professional.
2. Take out front seat. First, disconnect the battery...not necessary. Just don't turn the ignition on, you'll get an airbag light. Easy...4 T50 bolts. There is a connector underneath, just lift the black tab on the yellow connector. Insert a small screwdriver under the tab on the white connector...
3. Remove cover from outboard seat belt anchor. You need to be a man. Take a prybar and stick in the back like this...
4. Don't pry but twist the pry bar. You will hear a distinct snapping sound. That is normal for BMW interiors, they are well put together. This is what the little bugger looks like from the back.
5. Remove this bolt.
6. Get your harness, clip your lap belt together and lay the shoulder straps over the head rest...make sure nothing is twisted. Otherwise, you'll be repeating all of these steps
7. Install the outboard front anchor. You will need to bend the anchor outward, otherwise it will dig into the bottom of your side bolster unless you drive with the seat laid flat...like they do in da hood. I just laid the seat back flat, installed the anchor and pulled the strap to bend it.
8. Install the inboard front anchor. Remove the T50 bolt holding the belt tensioner.
9. The bolt does not fit in the hole in the harness anchor. I used a step bit to bore it out to fit. Doesn't take much. Replace the bolt.
10. The hard part is done. Reinstall the seat...x3 check that the shoulder straps and lap belt isn't twisted.
11. Install the rear strap...not hard. Again, T50 bolts. Replace the rear seat. Bend the achors as needed.