The right/passenger side brake light did not work and cause the warning "Light Failure". I changed the bulb and still it did not work. I then swap the bulb and the white socket from driver side to passenger side, the passenger side did not work but the driver side worked with bulb and socket of passenger side.

This means the bulb and the white socket is good, but it did not get the power and/or it has bad ground. How do I check and isolate the problem ? I have voltmeter. The wire connector has 5 pins, which pins are for the brake light ?

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'00 E430 Brilliant Silver/Ash