Thanks Bluesky. I had looked at that earlier but clearly didn't pay enough attention. It mentions the rust-colored wire as the positive, but didn't mention the negative and I didn't look closely enough to where the tap was.
I just tried it and it worked out well. Then the crimp cap came loose. I'm gonna have to go get soldering stuff and re-work a few of the wires now, b/c I severed the cigarette lighter (never use it & don't want it - at least not right there) to make room on the wire for this new connection.