Thanks for the strokes! I used Powerpoint, the basic Microsoft office software. I'll try to do a L7 version using the schematic info from the beginning of this thread, when I have some time. The HU to Amp part would just be a fiber-optic cable, though, with no separate wires showing, I think. I don't know too much about that one (not that I know that much about the non-L7, either, but trying to figure it out led to my doing the drawing because I get easily befuddled by masses of wires).
BTW, I'm still trying to find the HU output voltage. Anyone know? I saw 5V mentioned, but it sounded like a guess. My prospective installer wanted to know.