ignition light remains light.....!?

vauxhall cavalier mk3
ecotech cat abs elec windows aircon....
despite engine running and battery receiving a charge the ignition light remains on...the brightness of the lamp depends on the power used by the car...eg,,if i was to press in the cigarette lighter then the ignition lamp would go bright red....yet if i turn it off then it goes back to a dull glow. simularly if i turn the cars headlights on then the ignition lamp is bright again...all this despite the battery receiving a charge..this problem has been in effect for over 2 months now and the battery has never failed to start the car...fan belt is tight....no obvious defects......im at a loss...!!...any idea's..??

Ian T
October 2007