The isn't just One Earth point - there will be many of them.
Manufacturers use the vehicle's body as a return path back to the battery to save the cost and weight of wire.
The battery Negative often has a large short cable running to the bodywork - they call this Earth. A light cluster may have an Earth point possibly with a very short run of wire fastened to the bodywork.
Earth points are tucked away all over the vehicle and sometimes they become corroded or the fastening works loose.
People wiser than me often claim that an "earth problem" is the cause of strange happenings in motor vehicle electrics - I have yet to be convinced but have an open mind.
The way I see it is, that if an Earth connection is dodgy all it can do is prevent something from working properly - I just don't understand how a bad Earth connection can make something work that shouldn't be working.
When A is switched on and B starts working as well it smacks to me of a short or a wiring error rather than a bad earth.
As far as your indicators and Fog lights are concerned each Indicator Lamp will probably have its own Earth and probably shared with a light cluster. The Fog lights may have one each but they too may also share the light cluster earth points.
I don't beileve your problem is a bad earth but any earth should make a good contact with the bare metal of the bodywork and be done up tightly. Wire brush, sandpaper or just a star washer to penetrate the paint. If you scratch the paint away it's a good idea to wipe the re-made earth point with Vaseline to prevent corrosion.
Good luck...
Peccavi
August 2009
howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix vans.