Thanx for the details and pix of the remote wire source and rear channel wires!
You're fortunate with the touring that there's no isolation of the sub in the trunk, unlike the sedan which is very well insulated from the passenger cabin. If you put that sub into a sealed box, you would get even more awesome bass.
2006 330i | TiAg | Black | Al | 6MT | Sports Package | Logic 7
2008 335i | AW | Black | Al | 6MT | Sports Package | Logic 7
2013 328xi | Red | Gray | Al | 8AT | Sports Line | Premium | Executive | Heat Pkg