Std ford problem not just on sierra's.
The earth is a brown wire that goes in the multi connector going to each back light.
The pins on the rear bulb holders are galvanised but still rust and give these problems, these pins are the ones the connector plugs into from the loom.
You can buy new cheaply if needed new rear light bulbholders, each side, but may be ok with a clean up.
The best bet though is to make up a seperate earth wire for the lights/bulbholders, drill a 3/4mm hole in the earth track on the bulbholder, and stick a wire through and solder in place, then ring terminal on the other end to a good earth on body/lock bolts etc job done.
I always hard solder my light bulbholders in so don't suffer problems ever then.
tabetha