Flashing rear lights

I recently noticed that when the rear lights were on, they pulsed very slightly in time with the flashes.
I've had a look at the wiring diagram that suggests that there is a single earth from the rear light cluster.
Before I go and have a play tomorrow, I thought that I'd best check if that's the case, or if I will have lots of cleaning up of individual connections to do.

 

Yep. Check and clean the earth on the rear cluster. Classic fault.

 

Earth on the cluster, earth on the pillar beside the cluster.

 

Sounds far too straight forward!

 

I speak the truth. Do both sides. From memory, there’s an earthing tab welded to the bodywork on both sides.