Michael,

Thanks for the recommendation. I found references to the Sony XA-39 (MK2, II, ii???) on the BBS with further searching. Sorry for the hasty post. Now I just need one of these devices.

(edit: For future reference, though the Sony's are gone, some "source switching relays" are still out there. Precision Interface Electronics makes the MPSS-4 and it seems to be very similar to the Sony XA-39 II, and almost as cheap. Put "469MPSS4" into the crutchfield search bar for more info)

I already frightened one car audio installer with my idea so I'm sure I'll be back in here for help if I end up doing it myself (which I sure hope I don't, for the empeg's sake).