E30 tail light wiring / cluster maintenance

[gjm 3258]

The left (rear) reversing bulb on Mrs M's Baur is refusing to light. Swapping the bulb into another position - indicators, same purpose on the opposite side - shows it is fine.

The right reversing bulb does light, suggesting it is not a gearbox switch issue.

All other bulbs light correctly.

So, likely to be a cluster wiring issue. Anyone able to advise how to dismantle the cluster unit? There are retaining clips over a plastic panel, but even once the clips are released the panel will not come apart.

I'll check the wiring in the meantime. According to the diagram I have, it's the 'BU/WT' wire that provides the feed to the cluster(s) and a WT (white?) wire that provides power to the bulb(s). Earth is common.