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The normal causes for the problem you are getting is a bad earth caused by corrosion on the rear light clusters due to water ingress.
Remove the rear light cluster and check for signs of "greening" or rust on the interconnects between the bulb holders. Also have a look inside the light cluster connector for the same. If you find any corrosion be sure to remove it completey and then give it a shot of WD40 or the like.
Hope that helps a bit.
Currently in dave_mezza2006's garage:
2014 Vauxhall Astra Limited Edition 1.4 16v VVT Turbo