I own a 1977 300D which is giving me problems with the Glow Plug Circuit. About a month ago, I changed all 5 plugs along with the 50 amp in-line fuse and the ground wire. That started the car back up. However a month later the idicator lamp went off again and the car will not start. All of the plugs but one had power so I changed the plug - nothing. I changed the other four plugs (again) and the fuse - still nothing. One may say "check the timer relay" however everything in the engine is getting power. If the relay was bad, wouldn't that cut the power to the plugs? If it is the timer, where is it? I checked under the dash, but there's numerous relays and I'm not a mechanic. I need help!!