You have a flat battery.
The voltage reg is sticking and will run down the battery.
sounds like a bad light switch.
Check the battery. Turn headlights on - they will come on with or without the key if it has power from the battery. If lights do not come on, the battery is dead (try jump-starting) or there is a bad connection (loose cable), or a short in the electrical system. If headlights come on, but dashboard lights do not light up - check the ignition (cylinder / switch)
I had this problem on my disco 1. Turned out to be the alternator, which was replaced and the problem was fixed.
bulb no good.
Never unhook the battery with the ignition turned on. There is always a little voltage spike that occurs when disconnecting or reconnecting the battery. If the ignition is on the computer will be powered up and that spike could "fry" the computer.
When the ignition is turned on, battery voltage passes down the wire to the alternator to 'excite' the alternator when the engine starts. When the engine starts and the alternator generates a voltage, the light is extinguised by the charging voltage cancelling out the battery voltage coming down the wire. If and when the alternator completetly fails the the light will come back on as there is no sancelling voltage. if the alternator produces a voltage lower than the voltage from the ignition switch then the battery light will glow.
The light beam would leave the front of them at the speed of light.
Ever watched a movie called christine? Nah, check your wiring harness for the headlights and look for a bare spot(s) with wire showing...could be grounding
The headlights place a larger load on the battery and cause the voltage to drop enough to dim other devices slightly.