Originally Posted by elcheapoloco
Dull solder joint points to a few possibilities, most likely:
1. Not enough flux
2. Flux not sufficiently cleaned after reflow, causing oxidation.
Both will cause loose/weak solder joints. All you have to do is dab rosin flux on existing solder joint and apply heat with solder iron. You will see the solder joints turn shiny, immediately remove heat. Be sure to remove remaining flux with alcohol. Rosin flux won't cause oxidation as much as water-based flux, but it's acidic nonetheless.
Thanks for clarifying. They were dull from the factory. Not sufficiently cleaning the flux could cause the delayed failure. It's hard to say.BMW is collecting them and destroying them as often as they can on warranty.
For those who asked about getting them apart. I just used a small screwdriver and slowly worked around the lens mechanically forcing it apart. A hot knife or something else might be a better option. I don't have much experience at taking these apart. I'd lean toward a clear bead of tub and tile silicone for reassembly. It'd be water tight and could be pealed apart if necessary.
Like I said before, I cracked it the second time I attempted to take it apart. If you do take them apart, be prepared to crack them as it's very easy to do.