It is a metal box the size of your fist that has two sets of 4 RCA female chassis input connectors and 1 set of 4 RCA female outputs. It needs to be connected to the "amp on" or "antenna up" trigger outputs from both head units.
I set my CD/Tuner head as the primary so if the empeg is not in the car it just defaults to the CD player/tuner. It is made by Sony but I do not have the part number sorry. However, I believe www.crutchfield.com carries the device (you might have to call or write them) for US $49.00. I paid almost twice that, as a captive customer in a high-end audio shop ;)

Of course you'll also need high-quality metal double male RCA barrel connectors to connect the empeg's female outputs to the switch relay's female inputs. These were difficult to locate. I found them at a Radio Shack retail store. (4 for US$17.00)

Works great, just a minor pop when switching heads which may be a feature of my JBL amp.

Regards,
Mark Miller 20 Gig Green