After searching long and hard for a semi complete wiring diagram (for an aftermarket headunit install into an E90 w/ Hifi audio) and just not finding all the information in a single location, I decided to whip one up. I know some of you already know all of this but I hope this will save some people time in the future and maybe convince a few more people to upgrade their headunits.
This diagram was made specifically (with Visio) for my 2008 328xi with Hifi Audio but should work for most with the hifi package and an e90/92/93 chassis. The diagram shows the connections required to maintain steering wheel controls with an ASWC and the fuse location to connect to get 12V battery unswitched with no sleep timer (without having to run a wire directly to the battery). I recommend using an "add a fuse" to tap into the fuse box btw. I can also confirm that the ASWC auto detected my steering wheel buttons, all of them are functional. Please feel free to comment, critique or offer suggestions.
P.S. Special thanks to Garagemethods and whoever provided the link to the fuse box diagram
For people having trouble viewing the pics, click these direct links:
-> Wiring Diagram **Has been edited, changed from speaker level input to line level input (using RCA jacks on headunit). Results in much cleaner sound.
-> Fuse locations for 2007 e90
-> Fuse locations for 2008 e90, I used fuse 73 for my installation.
Here is a picture of my 08 e90 fuse box with 12V non switched battery locations indicated with red arrows and a link listing my findings and the fuse numbers:
Be sure to check YOUR car's fuse box diagram as the fuse locations change depending on month/year and transmission type. Look for the car door with the key inside of it symbol to see the fuses that are constant live (no sleep mode cutoff)