If you identify the earth track, I would just drill a small hole about 3mm in it, then solder a wire to it then bolt down to a good earth with a ring terminal on other end of wire.
The earth track will be the only one that is common to all the bulb holders, they are a problem on many cars, as with brakes lights tail lights indicators fog etc there is quite a load.
The above is what I have always done when needed, assuming the original metal of the tracks is galvanised like the fords, it's fine till a few years then micro arcing occurs burns off the galvanising on the conenntion pins leading to these problems.
I would suggest 14/030 wire would be fine.
tabetha