Hi FX35 -
I have a '96 300zx that has a 'Premium' Bose system. I do not know if Bose changed the way they implement Car Audio, but the system I have is Balanced audio (+,-,ground) from the CD changer to the head and amplified speakers. The benefit of balanced audio is high noise rejection.
Most Automotive audio systems and consumer electronics are unbalanced (+. ground). When connecting unbalanced and balanced components together balancing transformers are needed to make the system work correctly. The common symptom being loss of gain.
Because of these issues and also Bose's history of using poor/cheap components, most Z owners replace the Bose system as quickly as possible. I'm sure this is much harder (if at all possible) on the new systems like yours with highly integrated systems and features. In your case you will likely need a balancing transformer. See if you can find the pin out for your Bose connector. On mine it listed the signals like this:
Left front +
Left front -
Right front +
Right front -
ground
etc.
On some equipment you can strap the ground and the - together at the input and get some of your gain back, I would check with the equipment manufacturer if you can, but a transformer is always the better way.
BTW - I am having the same issue. I am solving it through different means. My "EQ" is a pro audio DSP and is balanced, so it interfaces well with the Bose speakers. I have solved most of my ground issues, but still have a very faint alternator noise I am chasing down.
You can read more about my system here:
http://www.mp3car.com/vbulletin/work...m-upgrade.html