Brake light in position 1, all lights fade in position 2

With the ignition in the first position (no lights) and engine running, the brake light and horn work fine and the battery charges (at least I believe so, see paragraph 2).

When the ignition is in the second position (lights) the brake light is dim and fades to nothing and the main lights are quite bright (normal) for a short period (this is why I think that the battery is getting a charge) and quickly dim to nothing.

Any ideas?

6v or 12v ?
If could be many things, bad earth, rectifier, stator......even a bulb I would change the rectifer and go from there