There are very few design flaws in the e46 (window regulators, sunroof guides). You just found one of them. BMW put dissimilar metals in the plug and pins in the tail light, and corrosion results. The ground connection corrodes long before the others. Cleaning it is a temporary fix. Replacing the tail light is a temporary fix. Adding a second ground connection is a permanent fix.
In the ground trace on the back of that tail light, you will find a tab for a spade fitting. Make an extra ground wire about 6" long with a spade fitting on one end and a loop on the other end that fits on one of the bolts that holds the tail light in. Do both tail lights while you are at it.
Actually, this was probably not so much a design flaw but rather a case where the plug and the tail light were made by two different suppliers who did not communicate their material selections to each other.
Here are the instructions with pictures.http://www.frontiernet.net/~papi/e46_Tai...
I looked at the way I did mine several years ago and noted that I connected my new ground to the existing ground wire rather than to the body bolt that holds in the tail light. That was a little cleaner than the way I described above.
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