Rear lights earthing point

I am having a problem with the rear light cluster on my GTA atmos. I have traced it to a bad earth. I can fix it by jumping an earth from the rear connector. But I want to track the poor earth properly. Does anyone know the location of the earth point on the chassis for the rear lights is.

Sorted now. I found the earth tucked in front of the block so it was in contact some of the time. Bolted up and all good. Silly, missed it when putting the engine back.