You can do a high output speaker level to RCA converter from the stock Bose subwoofer wires. I recommend using converterd from Soundgate. They are a bit costly but they are the best and they have one that will work for your needs. The unit is the LOC42
I like them because they are transformerless unlike the cheap ones. This unit will take 85watts input per channel.
LOC42 - SoundGate.com
I used one in my Landrover to add a subwoofer to the factory Harmon Kardon ICE system. I kept the factory deck and speakers as they were 3-ways in the doors and they sounded great for a stock system but I just wanted a sub for some bottom end.