gob

Hi My 2007 Zetec 2.0 TDCi has developed a fault with the electrics in the tailgate. We first noticed that the rear wiper had stopped working but then realised that the high level stop light and also rear window demister have also stopped working. I have checked all fuses under the bonnet and I can hear the rear wiper relays clicking when the wiper should be moving. Also we are getting the stop light fault on the dash too. I suspect this may be an earth fault on the tailgate wiring but I've no idea where to start looking for the problem. Does anyone have a clue where to start? thanks