Didn't someone, a long time ago, post something to this BBS about an off-the-shelf box which did precisely this? And if memory serves, it was even a Sony product and it did the job automatically (although I don't remember whether it used signal from the audio or the remote turn-on lead to do the switching). They even posted a picture of it.

Well, I am sure such a beast exists, but it would (obviously) be a digital device, with the switched being controlled in a fashion that the contacts would make no contact bounce, and be isolated from the unit themselves.

Now that you mention it, I do recall seeing something like this, and it even divided up the amp remote lines, per head unit.
Kinda like a KVM switch.
Well, KVM are great but none I know go above 1600x1200 with any sort of decent frequency (>75Hz)
So if the limitations migrate across to an audio device (let's pretend), then you would obvioulsy get frequency clipping on certain frequencies, but of which are probably not detectable to the human ear. (License plate rattle and window shattering are gonna happen regardless of frequency, its a matter of volume).

So perhaps such a device exists, but if it is made for precisly multiple head units in a single output environment, then cost would be rather high, as you get what you pay for.
All who use KVM's know that Cybex is the bee's knees for racks and servers and Belkin is good for cheapo desktops.

So, in conclusion:
sure, it probably does exist (nearly everything is nowadays, except for my hover-board....), perhaps Crutchfield might have some info?

My thoughts...

Smoker_Man
#080000449 MkII - 36Gb Blue