Adding subwoofer to 150W system

[Jaco2k]

I have the 150W system on my 2007 SC and am toying with the idea of adding the subwoofer in the spare wheel space.

Besides the obvious subwoofer and amp, what would be involved in this? Where to get the feed for the subwoofer from? It would be great if it would be possible to tap into the existing 150W amp to get some line output... since the amp is already on the trunk and the subwoofer amp would most likely endup right next to it.