This week I pulled the instrument cluster in my '85 300D to get the odometer repaired. As I was in the process of doing so, I forgot to disconnect the ground wire off the battery (BIG mistake) so every wire still had power to it. When I took the wire off the back of the tachometer, I accidentally moved it over and it hit the circular metal trim piece around the ignition key. After lots of sparks and some smoke, a fuse in the fuse box blew. Not thinking anything else had happened, I proceeded to take the cluster out, got it repaired, put it back in the car, and replaced the blown fuse. At this point I start up the car and everything works the way it should. THEN, as I turn the light switch on, the gas gauge needle slides to the "full" mark, the tach stops working, the water temp needle goes to 120 C, the lights in the cluster go off, the idiot lights buzz and flash randomly, and both green turning signal lights light up. The only thing properly functioning now is the oil gauge and the speedometer. When I turn the light switch off, everything is fine again. What could this problem be?