[ctuna]

Don't use any tapping . Get a 6 channel amp and Technic harness.
Or get a Technic Harness a forget about the rear channels.

The output of the Head Unit in the Hi Fi system uses differential outputs
which means the there is a a plus and minus signal with no common ground.

You have to have and amp capable of taking this kind of signal which is essetialy
a reduced speaker out.

You could tap the Inputs to the amp but I don't know why you would do that since
you have to disconnect the existing speaker wires anyway for the amp to feed
those locations.

BMW stereo types
http://www.e90post.com/forums/showthread.php?t=352586
http://www.e90post.com/forums/attach...2&d=1266451970
http://www.e90post.com/forums/attach...2&d=1260829447
http://www.e90post.com/forums/showthread.php?t=138949
http://www.e90post.com/forums/attach...1&d=1209780865
http://bmwcoders.com/forum/3-er-8/bm...1-e92-e93-291/
http://technicpnp.com/menuDiagrams/d...e90_92_93.html
http://www.musicarnw.com/page-2/
http://www.e90post.com/forums/attach...3&d=1169133185
http://www.e90post.com/forums/attach...1&d=1209780605
http://www.e90post.com/forums/showthread.php?t=641323
http://www.e90post.com/forums/attach...4&d=1278871872
http://www.e90post.com/forums/showthread.php?t=343673
https://www.bimmerfest.com/threads/e...n-752.1275694/