Like above. Use a bigger earth cable (at least 2.5mm˛) as short as possible from the lamp carrier to shell. If you don't find an existing ground point you can use a tap drill. The bolts for the taillight fixture will do the job, but there are better and more massive points.

The original cables are often undersized and will cause transfer resistance and then the plug-connector will melt after some time. No wonder, if you imaging that the power of ca. 5 bulbs (sometimes many bulbs glow at the same time) go back through a pathetic 0.75mm˛/1mm˛ earth cable.