New 12V battery discharges after two days

It depends.....on what is supposed to shut the power OFF to the amp when the car goes off.
My guess is that the amp itself is not responsible for doing that.

If it is an after-market amp that's been added, maybe the installer wired it wrong.

 

Possibly or maybe the amplifier unit is not getting the power-off signal from the head-unit. Did you try plugging the amp back in after you unplugged, just in case pulling the power on it reset something?

The other thing to try is to turn the audio off before turning the car off.

If none of that makes a difference, then you are probably correct thinking the amplifier unit is faulty.

 

No answers, but typically the wired current measurement on a multimeter will have a 10A slow blow fuse. So you can exceed that, maybe up to 20A, for up to 20 seconds or so before the fuse blows. So if you don't want to blow that multimeter fuse, ...

Maybe the answer was given in a previous post, but why were you surprised to hear music?

At this point, you have it fairly well isolated. Perhaps the easiest thing to check for is visually inspect the connectors - no corrosion that could be causing a short, bent pins, pinched wires where the seat may have done something, etc.

Beyond that, if you're trying to confirm whether the fault is in the amp vs. the head unit, you could analyze and understand the whole circuit (may not be practical) or you could try swapping a different amp in there as a test. Any good friends with the same amp? Junkyards?

 

I think all the evidence points to your amp being bad.

BUT....I also think it would be more than a little unusual for the amp itself to be responsible for shutting down when there is no signal coming in. Usually the power is removed by the main car shutting down via a relay probably.

 

I think this confirms that you have traced the source of your current draw.

To test the head unit, you only needed to put the car into ACC mode (one press of the Power button with foot OFF the brake; then, when you are ready, 2 more quick presses (again with foot OFF the brake) to turn off. In any case, going to any state of ON will increase current, and if you indeed put the car in READY, it will draw a significant amount in the initial start-up. The only real surprise is your Ammeter survived.

What your testing has shown, is that the amplifier is drawing current even when the HU (head unit) is off. This might mean the amplifier is not receiving the "Off Signal" or not processing the "Off Signal", or the other reasons outlined by 2012 Prius v wagon 3, particularly inspecting the connectors - no corrosion that could be causing a short circuit, bent pins. These things could be stopping the signal from reaching the amplifier. I am making the assumption at this point that there is nothing else wrong with the amplifier other than it is failing to turn off as commanded.

As ...

... this is good advice, and pretty much what I would have said.

I also wonder why you surprised to hear music, was it because you assumed the problem meant the amplifier would not work. As you have seen, this is not necessarily the case.