http://www.e90post.com/forums/showthread.php?t=52193
http://www.e90post.com/forums/showthread.php?t=352586
Threre is no fiberoptics between the Head Unit and stock amp(for the hi fi system)it uses differential or balanced signals. 5 volt +/-
There are Fiberoptics connections to accessories like bluetooth ,cd player from the head unit. There are many threads in the section that cover this
topic. The people that are expert in this are a little frayed answering the same questions over and over again.
Also when you remove the radio you loose alarms and some other stuff.
The door speakers are 4 inch with tweeters and 50mm deep. You can find all this stuff in the threads in this section.
Some people mount the oem head unit in the trunk so the car doesnt think its electronics have been messed with.