I am in the process now of adding a sub in my 19 M5 with Harmon Kardon audio systems. Planning on running a JL Audio 12W7E in a Pro Wedge Box that will sit in the trunk and I will run a remote audio knob near the driver seat for bass gain. Technicpnp make a connector cable that is plug-n-play and gives you the Sub wires out that can plug in a LC2 Line output converter. Adding a JL Audio RD 1000/1 behind the driver side panel above the factory HK amp it will be all hidden and look pretty. Can add some pictures once it's all completed.
Technicpnp BMW Connector
AudioControl LC2i PRO
JL Audio RD1000/1 (PDF)
JL Audio 12W7AE