Good question...I know M3 seats are very commonly used, and I've never seen anyone mention that they lack a locking mechanism, so I assume they don't. And if they don't then I assume the 650i seats also have an inertia lock...but that's a lot of assuming so I'll make sure to check
Either way works fine, but if it uses a separate module be sure to grab that and the wiring for it while you're there. I have a feeling you wouldn't want to pay BMW dealer prices on that stuff...
