Do any of the Sony XA-39 owners know if there is a "master" input on it? Anyone know where I can find a XA-39 now?

As Jim said, there are primary and secondary inputs. If both head units are on, then the primary input has precedence. Otherwise, whichever single input is on has access to the amplifier. If both head units are off, then the amp is turned off (assuming you use the remote amp turn-on from the Sony switch.)

I have two cars with Sony Switches in them. I bought one of then used -- was that from you, Jim? I think maybe it was...

tanstaafl.