Mystery solved!!!

There is indeed a bumper stop that goes inside at the end of the track
where the cam mechanism sucks the sliding door in tight to the body. If
you don't have it, the result will be a damaged door mechanism and
problably expensive to fix. That whole channel is attached to the body.
Looks welded.

Anyhow, ETKA illustration 107-00. Item #10, part number 251-843-876.
Cost $3.25 @ Camelback VW, Phoenix, AZ - 1-800-876-5199.

John Rodgers
88 GL Driver