Defo earth problem on the rear light boards, 100% std on any ford with a few years age.
I always run a seperate earth direct from the light board(bulb holder) to earth, as a minimum, then when I have time I always hard solder the boards in, 100% end of problems 100% of the time.
The problem is the pins on the board/bulb holder as std are anodised, although good for corrosion protection it is also a shit conductor of electricity, so soon there is micro arcing between the pins and the terminals of the block connector, this burns off the anodising so you get rust as well.
tabetha