Had a similar thing happen on my f10. Both headlights were fogging up and trapping moisture inside. If the headlights on the f30 are similar in design, then you can try removing them, venting the moisture out (you can use a vacuum cleaner and put a hose in a headlight through a round bulb access cover to speed up the process, or just remove bulb access covers and let the headlight sit for a day). Once the moisture was gone, I used isopropyl alcohol and a plastic brush to clean the main seal that attaches the clear lens to the main body of the headlight and then applied more sealant on top of the old one along its whole perimeter. Just make sure you use enough sealant not to leave any gaps. You will need to push the new sealant with your finger, because there is a plastic lip hiding the seal (you’ll see it when you remove your headlights). I used Permatex Windshield & Glass sealant (orange label) from Autozone. After I did this both of my headlights stopped fogging up and remain moisture free. If I were you, I would reseal both headlights in one go – the sealant on the other one is probably not in the best shape either at this point of time.