I don't know if anyone has seen this or not but I found this excellent write up and was able to complete this within an hour.

http://e30.bmwdiy.info/key-led/

I bought a CR1632 3V battery and bright 5mm LEDs at RadioShack, The final product ended up with a yellowish amber LED since the tips of the White LED broke off too short.

Much, much brighter and more efficient than that factory bulb.