The moisture will get in either through a small gap in the sealant between the two halves - run a thin bead of silicone around the edge - or a gap or hole in the rubber boot around the bulb hoder. Remove the headlamp, 2 minute job, if there's a lot of moisture you can get a lot of it out by pushing some kitchen roll in and shake the lamp around, then leave it in a warm dry place or in the sun with the hole upwards. No stone chips in the lens ? I bought a pattern unit for my 300 TDi from our Main Stealer, a Marelli unit much cheaper than OEM and it was identical - they'd even ground off the LR logo in the middle of the lens.
