Yep, it's a poor earth allright. Ford Escorts are another common place you''ll see that!
Basically the electricity can't get back to the battery by the easiest route, so it finds a way through the rest of the loom. As the bulbs' earth wiires are usually "commoned" up inside the fitting, the electricity ends up going via other bulbs, and Ohms Law for things in parallel, series and a combination of the two then applies!
Net result- random disco effects on your lighting board.
If you can find nowt on the lighting board, have a grovel under the back of your car - it may be the socket has a loose ./ broken connection