This is simple.... a dim light means a diode is bad.

I'm an ex-battery company guy, but... I don't know the specifics of MB alternators, but generally speaking, there are 3 positive diodes, and 3 negative diodes. If one is bad, the the alternator light will 'glow'. What it means is you are 'down' about 1/3rd your alternator power. You can drive it this way, and if you're carefull, it can even last a long way. Personally, I'd recommend removing the alternator, and having it 'tested' by a good alternator shop. They should find the bad diode, and charge you for just that. $30, and you should be back on the road. (now... can you find a good alternator shop)