You're not the first person who I've heard with this problem. Check the wiring from the bulb to the firewall. It may sound silly, but check the connections and make sure they are snug and not weathered to a point that they do not carry current. Most likely the wire has been damaged and needs mending. If there is no physical explanation then you may have a problem with your LCM (Lighting Control Module - I am not positive that the E36 is equipped with this). Occasionally this will create problems like one side is not powered.