The wires are actually closer to 20AWG than 18AWG.
But I just measured the current the amp draws at 12.5V, with only 2 x 4ohm speakers: <= 200mA. If it goes over 200mA, the music will become pretty loud that I think people outside the garage will know what song I am listening to. I am a bit surprised the amp draws so little current. Of course I will need 4 speakers at the end, but I start to think the current it draws will be well below 1A for my usage.
Next step I will measure the average current my NUC needs. With both cores at 100% it's about 1.8A, but I will need to wait until I get a 4G dongle and test again.