I have noticed this with many of the BMWs replacement lights for the older cars. It seems to happen more with the ones that have the glass bonded to the housing with a silicone type adhesive when they are constructed. The adhesive outgasses and clouds the back of the glass. The older ones were bonded with a more rigid adhesive and I never really noticed the issue with them. When those did get cloudy with age and use: they could be washed with alcohol as mentioned. The newer ones need to be wiped and then final clean with the alcohol. Some people have tried BBs in the alcohol to agitate and clean the glass but I have found that it is best to remove the bulb mounting flange from the light and use paper towels and chopsticks to wipe the glass. The bulb mounting flange is generally tabbed to the reflector and the seam is caulked over. this can be removed to gain better access to the inside of the light.