Well, the cheap easy way to do it would be to splice the wires going to the Bose sub and wire them to the High Input on the amp. If you cannot find the connector, you can always buy a High/Low level input converter and install it from the Bose subwoofer speaker wires, then into the amp.
I would not recommend using the Hi/Low level converter from any of the other factory speakers that the OEM Bose amp is powering. This is because the Bose amp filters out more of the "lows" from the speakers and sends them to the Subwoofer only. So, if you tapped into some other speakers, you wouldn't be getting as much of the "low" signal you'd like.
Or, for an easier approach but a little more money, buy the PAC Audio "AOEM-GM21A." It is plug and play. Keeps the Bose amp working, but provides two pairs of female RCA jacks and a remote amp turn on lead. This is basically everything you need in one little package.