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the tail light assembly is held to the car's body by three 10mm nuts (could be 8mm, but I think it is 10mm). Once you remove the nuts, one at the time, and make sure you do not drop them into the fender well on the inside, as then, you will have to pull all of your electrnocs out in order to fish out the nut, then you take the assembly out.
The assembly slides out, along side the main car axis, you will see, it comes off and goes back on just one way. The assembly might be stuck to the body by the means of debris, that has collected over the years, or just by simply, rubber sticking to the body... Just wiggle it out. when you pull the body out, you will see that the openings for the light bulb sockets have their metal portions, and those are the ones that are corroded. The LCM tell the driver that one of the turn signals is out by rapidly flashing the turn signal, and also by displaying a message on the high cluster.
clean the metal portion of the contact assemblies with a fine sandpaper, but keep in mind, you don't want the dust and debris end up inside the lens. wipe it off with alcohol wipe, not just wet napkin or whatnot...
the LCM checks the resistance of the circuitry when the lights are off, by sending a series of fast pulses to every light socket on the list, and reads the amperage of the circuit, thus checking for resistance of the line. If the line is broken, as in a burnt light bulb, there will be no current, and the resistance is infinite, so the light circuit is defective... But here lies the BMW engineering at its best - the LCM not only measures for dry contact, yes or no, it also measures for the current itself, therefore, if the line shows higher resistance, as if corroded contacts would, the LCM tries to compensate for it by sending higher amperage to that light bulb, which just extends the agony of the light bulb, and the higher amperage kills the light bulb rather sooner or later...
That explains, why sometimes we get the computer warning that the light is out, but everything is working - all it means, that the light WILL be out soon...
sorry for long winded explanation... lol...
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