Hi All
Just rebuilt my mk1 and the lights are driving me bloody mad! I can get on or two working or one side or the other but not all together. Pretty sure its just the earths on the rear clusters, (or lack of them as they just seem to earth through the mounting points for the units.
Has anyone upgraded ie run seperate earths, is it necessary from each bulb unit , or just from the light unit body

Most common problem is poor earths and or bad contacts within the fuse box/fuses.
 
First thing would be to try rolling over the fuses in the box with your fingers.  If the fuses are old, they may have oxidized? on the ends. Either replace or give them a light sand. Same goes for the fuse holders themselves, clean them up and make sure the pins have enough tension on the fuses.
 
Re reading your post, appears it is the just the rear lights?
 
Yes they earth thought the body.  Again, clean this up including the clips on the bulb holders. Seperate earths cant hurt.
 
Shaun

The bulb holders should be earthed when they go into the tail  lamp body, you have to make sure the body is earthed at where the nuts screw on the studs and the wiring loom is earthed
Jim