Oh well, looks like I'm going with the ferrite bead/load dump diode combination. There are a few cheap ICs between the car and anything expensive anyway. I'll post the results in a few weeks.
I'm working on a custom interface board to get my Tablet talking to my e46 BMW. It will allow be to plug directly into the stock radio harness and have provisions for iBus and the differential audio signals. The harness will also power the entire system. My question is, what kind of circuit protection should I place between the PCB and the harness? TVS diodes? Ferrite beads? Etc? How can I minimize noise on the audio signals? The Tablets audio breakout will be soldered directly to the board so I don't forsee a big problem with ground loops. Any advice is appreciated. And if it works, I'll post a full writeup. Thanks!
Ok so I ended up going with a 14V TVS diode and a ACM70V-701 common mode choke by TDK which says it's designed for car power lines. Also I have ferrite beads on all of my signals that talk to the harness so in a few weeks I should find out if it was all worth it.