Update! I'm going to keep this going in the DC wiring thread since it
is related to DC, just the intricacies of a particular part of the DC system.
I finally found a site with some information that helped me figure out what the problem was:
Brake light wiring with 3 wire turn signal help. | The H.A.M.B.
It turns out that even though the lights I bought are labeled as multi-function driving/turn/stop lights, they can only function as two of those at best - either driving and turn OR stop. And since they only have two wires (other than ground), you can't hook them up to three source circuits unless you use some fancy diodes or restrictors or something. So I bought another set of lights to use for brakes and used the first set for driving and turn signals.
I managed to find which connection on the buss panel is energized when the brakes are applied, but it was always hot if the brakes were on, so direct wiring to this connection would result in my brake lights being on whenever she was parked, even without the keys in the ignition. So that's a no-go.
I still had to figure out where to get the signal for the brakes, and the treadle valve on my brakes doesn't have any wires coming out of it at all, so that seemed to be a dead end. I followed a thin air hose from the treadle valve to the wiring panel, where it leads into a cylindrical device which has a set of four wires running out of it (no picture, it's so buried in the circuit box that I can barely see or touch the thing, let alone get a picture of it). I followed the wires to the computer for my KLAM electromagnetic brake retarder, and sure enough, they connected to the terminals for the brake light (among other things). One of the wires in that bundle from the cylindrical device did not connect to the KLAM computer, however; it doubled back to a relay and from there was connected to the switch in the control panel which turns the KLAM on. Which was connected to another wire which ran to a relay...
After some testing, I found that one of the terminals in that relay was only hot when the brakes were applied AND the ignition was turned on. However, as with everything in the circuit panel, the relay is rather inconveniently located, and I couldn't wire my new lights in directly. So I cut the wire I'd determined was the one I was looking for and spliced in my brake light circuit. The wire is only a #16 aluminum, which makes me a little nervous, but what else can I do? The brake lights work now, and I can have her inspected and (hopefully) get her registered!
What I still don't get is what happened to the original circuit? The wire I had labeled as the brake lights is directly wired to the buss panel, and is always hot if the brakes are applied - it lacks the relay which considers whether or not the ignition is on. I suppose that the eight-function flasher computer which I removed might have also been this relay; in fact, that's the only explanation I can think of.
The moral of the story? Label every single wire you cut, remove only what you have to or what is clearly useless, and expect that you'll still run into mysteries. Hopefully my "adventure" in this effort will help someone else avoid spending two months doing research and testing before they can fix their problem.
Cheers!
