you gotta get a voltmeter. if battery has decent charge (12 volts +/- .5), look elsewhere but you have to rule out battery having enough juice to turn starter first or else you are chasing your tail. also, my 300E glove box light only comes on when the lights are turned on - sounds like yours is different? voltage regulator is definitely something to check, takes 30 seconds to unscrew it from back of alternator and pull it out.

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2005 Mercedes C230k sport sedan, 6 speed
1987 Porsche 924S - 968-engined track car