The only info you get in VIDA (link here) is literally to “Remove the left-hand front seat”… and that’s it. At least they got this tiny bit of info right. You can check this post to see how the seat is removed. You actually don’t have to remove the seat from the car or unplug any of the cables. Just gently rock the seat back until the front of the seat lifts up and the back of the seat rests on the rear foot well.
According to VIDA, that’s it. You’re done. You’ve now replaced the HCM and you can put the seat back. But you see, you’re not done. The HCM is not just underneath the seat, but it is actually underneath a steel plate underneath the carpet. So, to replace it, you need to also lift up the carpet. To do that, you need to remove the sill trim at the bottom of the door.
Here is a view of how the general area looks like
You can see how the seat is tilted back. You can also see me holding the HCM cable. It was taped to the rest of the loom with a short piece of what looked like masking tape. Since my car came from the factory with the reflector xenons with no ABL function, I was surprised to see the the plug already there.
I think I read somewhere that the wiring loom comes with the HCM cable already wired in from 2007 onwards regardless of what headlights you have but I don’t know for sure. Would be great if anyone can confirm.
VIDA only shows a single low res image of how the HCM should be removed
In this ‘excellent’ image, the plate is barely visible (the transparent part) and the loom is missing completely, but they are very much there and very much in the way. The only way I found to make enough room to get the HCM installed was to cut the ziptie holding the wiring loom in place and gently lift it above and out of the way.
Even so, it took me about an hour scratching my hands and faffing about with the HCM trying to figure out how it’s supposed to go in, this is what I ended up with:
The HCM (or what is visible of it) is this part here
As you can see, it is obstructed by the wiring loom and this black steel plate or seat frame or whatever it is
Other posts seem to suggest that those plastic bits on the side of the HCM are tabs that should snap in place somehow. As far as I can tell, that’s simply not true. They are in fact just legs to keep the HCM off the floor. The only thing that I believe might be a tab is this bit here
Which I think fits like this
Even that tab, though, doesn’t really snap in place. In fact, the whole HCM just rattles about if I just look at it the wrong way. I don’t know how Volvo intended to keep the HCM in place and not have it rattle about. I think I got the overall location of the HCM right, at least judging by the crappy image from VIDA.
For now, I taped it to the floor with some electrical tape and stuffed some foam between it and the floor to prevent any noises.
So that’s where the HCM is supposed to go and how it looks like. I think. If anyone knows how it is actually supposed to be installed, do chime in.