My 1990 Syncro Camper does not have the oxygen sensor indicator light,
either. There is no mention of it in the owner's manual, and the Bentley
Manual does not have it shown on the instrument cluster wiring diagram for
1990, either.

The Bentley Manual wiring diagram for 1988/89 does show it, however. Maybe
you have a late-in-the-year production 89? According to Bentley, terminal
#14 on the 14-point instrument cluster connector is for the oxygen sensor.
Before doing anything else, I would check to see if your connector and
printed circuit sheet are both wired for terminal #14.

Michael