I installed a sub with my Bose system in my 2000 C-280. I tapped off the two rear deck speakers and ran them to the HIGH level input on the power amplifier. I grabbed the REMOTE ON off of the existing bose amplifier to turn on the amp. The color of the cable I used for the REMOTE ON was blue/white located next to the +12v and GND wires on the connector.
The two rear deck speakers, at least on my car, were already crossed over for low frequency
-B-