WRONG!

Nope, my logic on the colored wires in the KD-D50 was WAAAAAY off. The schematic still hasn't arrived, but I thought that with only 7 wires I could do some process of elimination work. I snipped the WHITE wire (by this time I suspected it might be LEFT), and I was correct.

Another cool thing: I disconnected RED and WHITE and hooked them to line-level R/L signal. It works exactly the same! That means all the spectrum analysis stuff is in the board attached to the black wire harness. The other lines must be power.

I had a theory that the LIGHT BLUE wire didn't impact the meters--I think that's true, but it shuts down the whole deck when it gets cut.

More updates later.

BLACK = some kind of ground--the whole deck shuts down.
YELLOW/PURPLE/PINK = power.