Trouble with Harman Kardon 2017 model

Hello,

I have no sound out from my infotainment. I guess some water corroded the amplifier under the passenger seat. So I bought an used one from ebay, and installed it today, but still no sound. When I move the volumen knob, nothing happens on the infotainment. However, in diag mode, it does say I do vol up and down.

So, basically, how can I say if the nav has a defect, or the cables going to the amp, or the amp itself, or fuse, or whatever? Any extra info in the diag mode?

Help much appreciated.