Are the top covers fitted properly? I used to get this occasionally with my previous car - I just waited for a hot dry day (unlikely I know!) then took the covers off and blasted the front covers with a hair dryer until they were completely clear, left the covers off for a few hours with the lights in the sun then put them back on. I didn't have any problems for the next couple of years after doing this.
2007 E92 335i SE auto | Quaife LSD | Birds B3 ARBs | Evolve Stage 1 | Morel Hybrid 402 | Rainbow IL-W8F | JL 10W3v3 | RF 3sixty.3 | Alpine PDX-V9