The hardware on some tether straps is simply too large to fit through the narrow opening Volvo put in the 2D and 2P head restraints. However, removing the plastic casing on the back of the 2D/2P head restraint can allow you to fit some tether straps through that otherwise wouldn’t fit through. You will need to do the following (as shown in the picture above): first, flip the head rest down, then from the trunk pull on and wiggle the plastic casing at the back of the head restraint (not the leather) and eventually the plastic casing will slide down and off, now you should be able to pass the tether strap through the head restraint. Note, that getting the tether back out may be much harder than getting it in.