A couple of weeks ago I got a replacement head unit for my car so I could connect my iPhone. I ordered from Crutchfield, I highly recommend them because they do provide all (well typically) you need for an installation.
After I installed the new head unit, I had noise in the system when the key was on. It did not matter if the car was running or not so it was not alternator whine. Crutchfield at first suggested a ground loop isolator but I also was not getting the same sound level out of it as I was before so then I tried a line out converter to use the speaker outs. This worked okay but I was not satisfied with using a bandage on the issue.
Finally I found that the Bose Amplifier wants a balanced input while the head units puts out unbalanced outputs. Check this link to see what I am talking about Balanced Line Drivers
Using unbalanced outputs cuts your output in half and using balanced outputs eliminates noise.
Searching online I could not find a reasonable balanced line driver (unbalanced to balanced converter) so I made my own! TI has a chip I used http://www.ti.com/lit/ds/symlink/drv134.pdf
To power it I used a DC/DC converter and a couple of linear voltage regulators.
It was a fun project and it sounds great.
Here is the PCB (not my cleanest work and it is before I cleaned the flux off, but it works).