Volvo xc40 amplifier

[Lukas]

That is strange, both AUDs shows the same SW part and configuration numbers. 
What is you OEM settings for 028 ? 
If it's 01, put it to 02. 
If it's 02, (less likely) put it to 04. and reset the IHU and AUD. 
If still you cannot hear from pin 02 (-) and 09 (+), either your IHU does not have the software to enable the AUD or the AUD you both is faulty. 
Send an email to support, they might have the pre-amp software for your IHU to enable it.

OEM 028=1.  that was the original.  I wrote for support.  so if I can, I'll install the software.  I hope it helps.

[TheKris79]

Also worth to check if maybe even the lower level amplifiers have the speaker resistance check function (apoc). If so then I saw that you only check with 1 speaker connected to your output, but if you have the speaker check function in the amplifier then it sees there are speakers missing and shuts down the outputs. In your case if 1 rear speker is missing it will would cut off both rear channels.
I have fiddled around with my 2014 s60 premium sound system and was also fihting with it due to this issue. Then needed to add an Helix AAC conversion to activate the original Amplifier outputs to get sound (needed to get inputs to my new amplifier through the existing one).

Just worth to check and it should be easy to connect 2 speakers instead of 1.