Just wanted to follow up and kind of close the loop on this. I was installing the new stereo in the dash, and at first was going with the standard wiring - one circuit that's always live (for memory) and one that comes on with the ignition switch. Then I realized that we might want to use the stereo while parked, and wouldn't necessarily want to turn the ignition switch on. Well, turns out I have this yellow wire coming from the house batteries, running up under the dash, and just about the perfect length to run the stereo. I suspect that's what it was for - if not, that's what it's for now!
