in the c230 2002 headlight low beam conector are 3 wires 1 yellow and 2 brown wich one is the positive and wich one is the negative

 

I couldn't tell you off hand, especially since I'm new to MB cars. What I would do is use a dc voltage tester to determine which is which. It's a very basic proceedure and you can buy a decent digital multimeter for a small price. You can buy them at any hardware store, radio shack and many department stores. It's a great addition to you tool box and you will use it in the future. All you need to do is touch the black lead (negative) of the multimeter to any unpainted metal of the car. Then, with the lights on,stick the red lead (positive) of the tester into each wire of the connector of the light until the tester reads 12 volts. That will be the positive wire.
If i'm unclear or you need further explaination, just ask... There are no "dumb" questions.