I saw the same thing on mine when I was changing the lens to grey/red, it actually has no bulb in it while the otherside has the contact and a bulb. I have no clue what it is for. As my car is a Euro I figured it was just some wierd Euro option. I am now wondering if I could do some soldering and turn both sides into extra brake lights but I have to know what their purpose is first.......anybody.
88 230E (W124)