It might well be a dodgy regulator unit on the alternator - mine went this time last year, symptoms were:
Battery light glowed faintly all the time
Occasionally the rev counter would go mad, flicking up and down. The battery light flashed in time with this.
If I stopped with the lights/radio on for a while, then started the engine and drove off, the light would be completely out and the rev counter perfectly behaved.
