Isn't there a way the empeg can sense when the aux is on and turn itself off ?

First of all, I've never heard of a head unit that automatically sensed when the Aux input went active. If the Empeg had this ability, it would be rather unique, I'd think.

If the Sony unit has an Aux input of its own, you could wire them the other way around. Then whatever button Sony uses for that purpose would be the one you pressed. Probably simpler than digging through the menus.

Another way of handling it would be to wire BOTH units directly to the amplifier via an A/B switch of some kind. A DPDT switch would do it (assuming you didn't care about fader control). It might even be possible to use a relay instead of a switch, and have the relay fire only when there's power flowing to the Sony unit. Not quite sure how to handle the amp turn-on lead in that situation, though.

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Tony Fabris