Yeah, I saw that wiring diagram. It didn't offer much help.
Anyway, by trial and error I found the correct wire (an ugly brown one in case anybody ever has a similar problem) which was poorly spliced. I stripped the wire, re-connected it, taped it up and it works perfectly. I ended up working well past daylight (by use of a flashlight and the reverse lights on my car) and got it mostly put back together and took it for a little test drive. All seems good for now but morning will tell for sure.