You may just need to replace the lamps. Also note that if the lamps were replaced before, they may have the wrong bulbs because the BMW computer has the wrong part number - it lists a 6V bulb.
I changed mine about a month ago and now I need to do it again because of this error. Go here for instructions:
http://www.nmia.com/~dgnrg/page_2.htm
The guy's name at this site is Gale. Make sure to ask for the BMW part number that Gale lists when you buy new bulbs - about $1.00 each.