hey dude well it might sound dumb to some people but i had a glow plug light problem on a 240D before and i thought it was the ignition switch but it was not! it was the Battery!!! i changed the weak battery and now the light comes on! MB told me if MB diesel's have a the wrong battery in or a battery that was short circuited the glow plug light will fail but you still can crank it! weird huh? well i learn something new everyday check your battery man! check your key switch, GP relay, fuse on the fire wall and the wiring. ill say it again it happend too on my w123 300D to!! i had to change the battery because the GP light would not come on! weird huh but it was true for my car and my custumers car! cya....

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1991 560SEL AMG "Jap spec"
1989 560SEL Euro spec