CRuby

Back up tail lights stopped working on my son's 2000 A6 2.7 6MT
Traced problem to the backup (BU) light switch mounted near top right of transmission, just near the PS front O2 sensor. Jumped the 2 wire connector & BU lights come on. So wiring to lights, & lights are OK. Removed the BU switch from the trans & its an open/closed plunger type switch linked below. Checked continuity of switch & it works fine. Barely have to push the plunger in for circuit to close & good continuity. I inspected the plunger & it's not worn or binding up at all.
So, I'm thinking the plunger switch is no longer getting pushed in when shifting into reverse. There must be a lever or such inside the trans that is supposed to push the plunger in when shifting into reverse. Is it possible this lever is worn, or out of adjustment? Attached below is pic of trans location where the BU switch goes.

Is there any adjustment on the trans for pushing this plunger on the BU switch? There is a large bolt/nut just above the BU switch location, is this for BU lever adjustment?

CRuby

Also found this pic in Bentley, but still don't know if reverse linkage can be adjusted. "C" is the BU switch.