Just going to add what I learned last week replacing the battery in my Dyson V7, not the same but looks very similar. I noticed that the trigger switch moves a linkage that presses a switch on the battery itself. It seems that the switch is built into the battery which is really hard to break into. Not sure if its the same for yours just something I learned (kind of a clever idea) that I though I would share.
If its not pulling in at all I would think it needs to be replaced. The mechanism is a complex little part that can even have a small charge that fires with the air bag to tighten the belt a little. When you pull the belt all they way out dose it make the typical seat belt noise? Then its just all out sitting there with zero tension? I had a belt that would retract slow or only 90% of the way; I washed the belt in hot water to make it more flexible, that helped a lot.
I have a different type where the trimmer is an attachment. I was able to open it up by prying along the split line, then I can clean and sharpen the blades. This type looks different though. A nice close up picture might help
I had this issue with my 2016 Impreza. After pulling apart the center console, I figured out what was causing my issue. To move the shift knob the driver needs to squeeze in the button on the front (blue circle in the picture). This lifts up a rod through the knob (red line in picture) . with the rod up the shift knob can be moved into gear. The rod also presses a button when in park, releasing the key. My issues was the rod was not dropping all the way down and activating the switch. I pulled the rod out and put some light grease on it, hasn’t been a problem since. It was not to much work to get the rod out but I don’t remember exactly how. Sorry its so late, maybe this will help someone else
