There's a how-to published on volvoamazonpictures.se but it's probably the one you're refering to since it instructs fastening to inertia reels in the trunk. The problem with fitting the reels inside the car is that the only place to put them is on the shelf behind the backseat which requires two things:
1) the inertia reel must be designed to release the belt also when the reel is mounted in a lying down position (as is the case on the shelf), which far from all reels do, and
2) the shelf must be reinforced in order to be able to withstand the strain of an adult during a collision (many states/countries have mandatory inspections of cars and the car would not pass such an inspection without this reinforcement)
In addition to this, the reels will be visible on the shelf.

Brgds,

Fredrik
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