Hi there hope i can be of assistance.
Ok, this is likely to be a bad earth in the local area.
Each cluster will have a wire running from the multiplug, to the body. Its highkly likely this wire has either rubbed through, or the fixing bolt has come loose.
So remove the light cluster and have a good wriggle/quick pull oin the wiring to make sure all is secure.
Also remove the multiplug off the rear light cluster, check for any small signs of corrosion to the plug itself, the pins on the cluster, and if visable, check the platic/metal molding running to the bulbs for any black/green signs of corrosion.
If know this sounds unlikely to be corrosion, but if there is a poor seal in the boot area, or at the light cluster then corrosion can quickly occur.