Feeding amp a 12v turn on from LOC

My standard audio Toyota Prius head unit has no live 12v amp remote turn on.  There is a pin for it but no wire from the harness for that pin and apparently even adding a wire won't work because the pin is dead.  It has an antenna on but (as usual?!) it only works when the radio (but not the CD) is in use.  The head unit also lacks line level outs.  

So I purchased a PAC-OEM2 LOC which can also generate a remote turn on when it senses audio coming through any positive speaker wire.

It's been suggested that that method might not be reliable and also might introduce grounding issues.  (It needs to be fed 12v acc power and be grounded but only if that remote trigger function needs to be used.)

Was wondering if anyone has had good or bad luck using such a device to generate a 12v amp remote on and if not, what are typical methods of getting a 12v remote signal to an amp in the back of the Prius.

It usually works fine (and I've never seen it cause a ground loop) except for one thing: in most modern vehicles the "radio" is actually interfaced with the car's data bus system, and there are many random signals that get generated into the speaker leads from the data system.  What usually ends up happening using a speaker-level switch is you cause the amps to turn on and off randomly when the car doors are opened, the automatic locks activated, etc.  Generally if the radio is on these random signals are not detectable and cause no problems.  It's when the radio is off that you get this effect.  I had one friend burn up his amp this way: he drove several hours with the radio off, but the PAC switch on the speaker leads had activated his amp.  For some reason the amp overheated and there was no signal going throuh it (and this was a decent Planet Audio amp.)  No real idea why except for the on-signal from the interface. <<shrug>>

So, yes it will work, no it shouldn't cause noise issues, and yes it may caiuse other strange problems.

Yes, one such consideration is the touchscreen beeps routed through the audio system, even when the radio is off.  By the way, those beeps reportedly get amplified to an obnoxious level when an amp is added - something else the PAC-OEM2 can apparently fix using its output adjustments.

Fortunately, it seems the beeps (and hopefully everything else) on the STANDARD Prius audio (but not on the premium) are routed ONLY through the front channels.  That being the case, I'm hoping hooking up the PAC's sensing wire to one of the rear speaker wires will solve that issue.

Also, wouldn't the alternative be an even bigger risk for the problem you mentioned: hooking up the amp's remote line to a 12v acc in the car would mean the amp would ALWAYS be on anytime the car is on, dramatically increasing I'd think the likelihood the amp will be on without the radio on.

The risk the amp will burn up under that circumstance is of course important to me but also how much electricity is used up by the amp when no signal is sent to it (radio off).  It'll be the aux battery on the Prius that will power the amp. It only supplies 80 amps total.

These are the grounding / reliability issues brought up and the threads where:

"... however this may introduce a ground-loop if you ground the line-level convertor at a different potential than the headunit. If you put the line-level convertor at the back and it requires a ground, you should also run a ground wire to the deck instead of grounding it at the back with the amps."

(Not sure what was meant by the last sentence as I had no intention of grounding the deck anywhere other than it was at the factory.)

https://www.the12volt.com/installbay/forum_posts.asp?tid=53306&PN=2

"I don't like trigger devices that sense signal in OEM speaker wires, and I don't like to ground a device that's a part of the signal path if I have an alternative.  So if it were me, I would run a remote wire to the amp from a switched power source.  That wire would be fused (1 amp fuse) close to the power source.  The amp will be on when the car is on, off when it is off.  The LOC will act like an LOC and not be depended upon to trigger on the amp.  And you will have one less ground plane....because as you know....or will know by this weekend if you study BCAE1.com....ground planes are the primary reason for noise."

https://www.the12volt.com/installbay/forum_posts.asp?tid=53350&PN=1