This is how I hooked up a powered subwoofer to my non-Bose 98 A4 Concert stereo.

• Run main power wire to the battery and route to the trunk. You can lift the trim moldings slightly and push the wire under. I ran the wire through the ECU case. There is a 4 inch by one inch hole that the computer wires follow into the cabin.
  
  
• Connect Ground to an exposed screw or other joint. Be sure to use the same gauge wire as the power wire. The left rear speaker contains a small circuit board which drives the rear speakers. Disconnect the circuit board from the factory harness. There is a small release button on the blue connector you need to press. I removed about an inch of additional tubing from the wire bundle to have more room to work with. I used 18-22 gauge crimp-splice connectors I got from Radio Shack.
  
  
• Connect the white wire to switch on/off the amp. Do NOT use this wire for the amp's main power. Find an old pair of RCA cables and cut it in half. Strip three inches off the end of each lead. Twist the outer bundle shielding together on both wires. Trim the common ground bundle to about half an inch. You will now have a wire with two RCA jacks on one end and three wires on the other end. (Remember, one for each signal [Left and Right] and one for both grounds).
  
  
• Connect the common ground lead to the brown wire with red stripe.
  
  
• Connect the Left channel wire to the blue wire with green stripe.
  
  
• Connect the Right Channel wire to the blue wire with red stripe.
  
  
• Connect the RCA jacks to the amp's pre-amp input.