Success! I was able to tap the inputs at the Bose sub with an RCA cable. No line-out converter is necessary.
At the Bose amp under the passenger's seat, you'll find three harnesses. The first one, the one on the left, are the speaker inputs. I tapped off the wires on the right on the plug, red/black, white/green. I did this with the red quick splices you can find at Radio Shack.
I could never locate the remote turn on wire at the bose amp, so I added a circuit to the outlet fuse at the interior fuse box and ran a remote wire from there.
I selected an 8 inch sub in a ported tube enclosure, and I'm very satisfied with it. I don't want to rattle the neighborhood, just have a foundation of bass, and this fits the bill. The bose woofer in the tire well just didn't bring it, and functioned more as a mid-bass driver. While I'd toyed with the idea of changing the bose door speakers, I think the addition of the sub makes all the difference, and I think I'm done.