You can't take it from the Low Level full range signal without introducing some nasty engine noise. The easy way to do it is taking the High Level Speaker out and summing the signal.
One thing I've been wanting to try but haven't got around to yet is splitting the full range low level signal while keeping it connected to the Bose AMP and splitting off into an RCA plug. The Bose AMP works with the bose headunit using a Balanced signal and cancels out the noise. I'm hoping by splitting and not completely disconnecting it will keep the noise out but I don't know if it will work.
So unless you want to try that and see if it works you cannot take a clean low level "full range" signal without introducing some horrible engine noise.
Anyhow to answer you question here's the Low Level wires- They aren't like speaker wire they are shielded and with the negative and positive twisted together much like RCA line without the rca plug.
