Quote Originally Posted by AllanS
If it's the part that I'm thinking about, the one that moves up and down along the pillar trim and holds the belt,
It's the rear seat, at the top of the rear seatback. There is one embedded in the leather on each side. The belt feeds into it as it diappears in behind the seat and apparently you have to remove the seat and unbolt the belt (somewhere) to thread the new guide onto the belt.

If it's in Bently, or someone has instructions, I'll do it myself.