am having a problem with rear light cluster. when i brake the lights dim and the indicator fails also get a message on the dash telling me the tailgate is open. problem is worse in the winter so thinking it might be a bad earth somewhere but dont know where to start.

Check battery terminals, check the battery itself. If that is all good, check connections on the passenger fuse box are all in. It might be your battery as you say it worse in winter, car electrical don't really get worse if cold or warm unless you have water issues etc. But check battery.

thinking it might be a bad earth somewhere but dont know where to start.

I think you are on the right track. This sounds like an earthing problem. What happens is that if the earth fails the current flows back to earth via the lamps in the other cluster. There should be an earth wire from each cluster to a point in the boot. I can't point you to where but maybe one of the gurus on here can. The wire may be broken inside the flexible gaiter between tailgate and body.

Agree with the earthing issue. as mentioned, check the rear gaitor to verify it hasnt got a broken wire. Also worth checking the earth points for the light clusters. On my mondeo, and also on the CMAX before it, I would remove the rear plastic panels for the boot wall and the earth point was there, worth checking its connected, clean and secured on the side with the fault, and following the wire to ensure there is no break in it.