It is never a bad idea to create a new earth from rear lights units, because BAD EARTHS are the cause of almost all problems in back lights.
The moment you see a rear flasher winking dimly, and the other lights winking dim when the yellow one flashes off, you can be 99.9% sure there's a bad earth.
Find the earth tag in the lights unit. Clean it and apply a little clean grease to it.
Get a nice hefty black cable, and SOLDER a new push-on connecter to it.
Push that onto the earth tag of the lights unit. Now lead that cable to a nice clean dry metal part of the body or chassis, and basically, BOLT IT DOWN.
Job done. Now do the other side just the same. They could share the same body/chassis earth point if you were smart.
The rear lights may appear brighter after this job, which will be because they ARE brighter due to a better earth with lower resistance.
CharlesY