Hi,
This is my first post in this forum and I am no car audio expert at all, so please do forgive for my language.

My problem is the following:

I own a Volvo V40 car. It has 2 front speakers, 2 rear speakers and a subwoofer that is coupled to the rear speakers. That is: the level of the subwoofer is controlled together with the level of the rears: rears louder means subwoofer loude, too.

Perfect for me! That was with my Alpine CDA 9853R.

Now I switched to an Alpine iDA X001.
I unplugged the "MultiPin" connector from the old 9853R and put plugged it into the new X001.
Everything works, except for the subwoofer!
How is this possible? How can I change this? I don't want to use a separate amp for the sub, I like the sub to be controlled with the rear speakers.

Can anyone help?

I would check to see if the alpine wire harnesses are different. If the subs worked with the other alpine there should be no reason for it not to work with the new one. The wires I would be looking for to be different or not there are blue and blue white ones. These are the pwr antennae and pwr amplifier respectively. The sub probably takes it signal off the rear speakers, but is powered by an external amp. You could use an accessory wire for an amp turn on. If you say there is no amp running the sub and all of your equipment runs off the head unit amp chip then I would say it is not powerful enough.

Your subwoofer is a passive or active subwoofer?