406 Mk2 Window replecement

Following a recent break-in I've had to replace my front passenger
window (2000 Mk2 model with electric windows). Whilst removing the
glass and the seal going round the door, I came across a small white
plastic part. It's about 40mm long x 6mm wide with what looks like a
plastic rivet going thru' a hole in the back. As the replacement window
goes up/down the window drops out of the door seal so I'm guessing it's
some sort of guide.
Any pointers as to where it goes and how to re-fit it?

 

This guide attaches to the rear of the window via the rivet (there's a
hole in the glass) then pushes into the rear window guide.

 

Cheers nigel.
I asked the breaker for a MK2 window and got one complete with the
cresent moon-shaped bit in the hole. When I compared it to the driver's
side which also had once in I assumed this piece was extra and not a
different version of retainer. Removed the smaller piece of plastic and
put in my larger channel runner - but not before I broke the damn thing
trying to get the rivet out. Thank the gods for superglue