2014 Regal GS with HID lamps. Water collected in the driver side housing. Started with just some condensation, then after some time including some rainy days and a car wash or two there was more than an inch of water in the housing. Did some reading online and here are a few things I learned while researching and fixing the issue.
1) New housings are expensive. A HID unit (the entire housing) for one side was $950 or more. If the car ever had front end damage from an accident where both housings require replacement I bet the insurer would total the car. Regular housings (halogen) were much cheaper but I do not believe they are interchangeable if you want to keep the Xenon HID lights.
2) Removal was not a big deal. two electrical plugs, three screws in the wheel well and six or so across the top radiator support provides enough space to wiggle it out. The front faring does not have to be removed from the vehicle. You don't even have to remove the wheel, just turn it to the side. Two different size torx bits are required.
3) I poured out the water, dried it as best as I could with a hairdryer on low heat and tested for the source of the leak. Drilling a pin hole in one of the access covers and applying low psi air immediately showed me where the seam had lost it's seal. 
4) Someone suggested silicone with a syringe. I call a party foul here, there is no way silicone will flow through a needle. I actually tried it. Possibly a baby medicine plunger, but I did not need anything of the sort. I cut a cone for the silicone tube to it's finest point and opened it up a bit with a medium size nail. Had no problem getting the silicone where I wanted it although it does take some hand strength to make it work. if I did it again I would use a caulking gun over a tube. The silicone filled in the space between housing halves and dried but did not make a good seal. After testing with air the next day the leak was better but still there. I cut out the silicone and used 3M super weather strip adhesive, this worked awesome. Wish I had started with this stuff. Sealed the pinhole in the cover with a piece of tape and made sure the one with the hole was not on top (water collects on that cover due to drainage design).
5) The bulb was not working, so before reinstalling I swapped bulbs. No luck, swapped the ballast to confirm it was the ballast that had shorted. Note HID bulbs are $130, and a replacement ballast was $30 on amazon.
6) All fixed for about $50 and a few hours of my time.