I don't see why there is so much confusion. You are simply extending the connection between the oem radio and the harness in the dash (which would normally be plugged into the oem radio).
Pins 9, 11, 15, 12 and the fiber optic cables need to be extended. it's as simple as that.
One metra harness connects to the oem radio and the other to the oem harness in the dash. So you don't have to cut into any factory wiring. Simple as that.
2009 E92 335i M-Sport TiAg|Black|Aluminum -ZMP|ZPP|NAV|CA|Heated Seats|6MT
BMW Performance Exhaust|Rear Fogs|OEM Black Grills|Cobb AP|ETS 5" FMIC|DCI
Installer for BimmerTech