I have now been able to troubleshoot this further.
The frequency of the left side cutting out has increased in the last weeks.
It has cut out the left side a bit longer aswell, I put the fader all the way to the left when it did.
There actually is some music still playing, but it's very very low volume.
I'd say it's like when you press Mute on some headunits it actually only lowers the volume by 20dB or so...
This tells me that the cables are fine.
I contacted my place of purcache, they asked me to run the system without the amplifier just like you Nico.
Of course that is easier said than done but I took some measurements with a multimeter and jumpered the Alpine kit's cable harness.
It's not pretty but it works, and above all the sound no longer cuts out, so that confirms the problem is with the amplifier...
So I will be getting a new Amplifier in the mail soon