Well, it works, at least with my hacked temporary test setup. I took the newsgroups advice and wired together the 12V constant and switched lines on the empeg, then connected them to the 12V constant on the car. Thus, the empeg is always on even when the key is out (which the stock unit sort of does anyway). I wonder if the amp is always on too, which is unacceptable.

Now that I know it actually works, I will run a proper switched line somehow... I have no idea how. Maybe I can run proper illumination and dimmer sense wires too.

Luckily the speaker wires matched the VW wiring harness; even the polarity. I had some good suggestions to confirm. Could I have used the battery trick even though the inputs go to a Bose amp. Still, it would require the speaker grilles off to watch the cones. It sounds good, so I'll trust that it's right.

Thanks again for all your help, everyone. Now to buy the components for the "master plan".