A common question I get from my site is: "How did you add a subwoofer to the factory Bose system? The guys at Xzy Autosound said it can't be done. Please help."
Not entirely true. Almost every auto stereo place I've been to has stocked hi-level to line-level converters. What this handy item does is take the hi-level power from your system (what runs to the speakers) and steps it down to the low-level signal that most aftermarket amplifiers accept for input. You simply connect the converter to the speaker level source in parallel (I picked mine up at the speakers) and run standard RCA-type patch cables from the converter to the amplifier inputs and all the necessary power for the amplifier. Also, some amps will even accept hi-level speaker signal as input so you don't even need a converter. I was initially concerned as I'd heard Bose amps were very sensitive to impedance and I wasn't sure how it was going to work out. The load presented by the converter is pretty small and it does not (at least on mine) affect the sound of the tapped speakers one bit. I ran this setup for 2 years in my A4 and and currently on the system in my S4. The factory Bose amp does not seem to mind one bit.
It bears mentioning that I would not recommend using a converter to run anything but a subwoofer. They do not yield the quality signal you would get from a true line source. Main speakers running off a converter usually sound somewhat harsh with compressed dynamic range and do not yield the clarity that a good head unit would offer, but for running a subwoofer it does just fine. Refer to my frequency response testing for how mine performed.
Don't forget to get one of reasonable quality... if it costs $6.00 it will most likely perform like crap-o-la. Mine was around $24.00.
Quick and dirty install info:
I ran the 4 ga power wire through the firewall. If you look at my washer defeat switch
installation you'll see the location of the red 4ga wire. I removed the dead pedal
and ran the wire under the carpet, then tucked it up under the door sill covers (I
couldn't get them off), under the rear seat, then up the front edge of the LH rear fender
to the underside of the deck where the amp is mounted. Yes, I have my amp mounted
under the deck. No heat problems... even with me CRANKING the amp in the summer with
100+ degree Houston weather. I've yet to have a thermal shutdown. Your amp may
not be so forgiving so choose your mounting location wisely.