BMW Repair: rain sensor 2000 540i sport, crackling sound, rain sensor

Question
I recently acquired a 2000 G.I. sport , at first the intermittent wiper worked well at position one on the wiper lever later,it did not.The wipers work but when i set it to the rain sensor position the wipers work in a low speed the same as position two but no intermittent action. While was driving in a light rain storm i heard this strange crackling sound it seems to originate from the small vent on top of my dashboard on the driver side close to the windshield. There is not any smoke or smell when i look under the hood it seems to be under the filter on top of the engine,drivers side I only hear this when its raining. I recently had the car detailed if they put rainex on the windshield could that affect the rain sensor ,or is my problem mechanical or electrical

Answer
This is a strange problem, Im not sure what the noise is or fi its even related to each other, Has the windsheild ever been replaced look at the bottom of the windsheild and see if you can see a BMW emblem or the words BMW in the glass,if it has been replaced with non BMW glass the rain sensor will never work correctly.     hope that helps.......Thanks Nathan Babbitt (Babbitt Motor Werks)     p.s.   did the rain sensor ever work