Sounds like you're trying to set up how I had my old install wired up.

You need a 12v relay with some NC contacts - Jaycar do one that's suitable and the same as what I used.

https://www.jaycar.com.au/12vdc-dpdt...tacts/p/SY4008

Run your amp remote through the set of NC (normally closed) contacts, ACC 12V (ignition 12v) to one side of the coil, and the blue ALP mute wire to the other side of the coil.

When it alerts, the coil of the relay will be energized, the NC contacts will then open, dropping out your amp power.

As for any other way to do it, without a mute input the relay will be easiest