Low voltage and weak performance is consistent with a bad diode. But that doesn't explain why your light stays on without the key. For that to happen, one leg of the bulb would need power and the other ground. A shorted diode would allow power to leak to the D+ side, but the other leg would have to find ground somewhere, and that leg leads through the ignition switch. It may be a normal quirk of the Mercedes dashpod, we'll see what happens after the alternator problem is cleaned up.