[mcconnell45]

I'm going off of my '03 silverado for reference but I would imagine your Sierra isn't too far off.
When you look at the headlight with the hood open there should be a metal rod sticking up on each side of the headlight assembly. Unscrew these rods until you are able to pull them completly out. Once you do that you should be able to take the headlight through the front of the grill making it easier for you to either unscrew the bulbs or just unplug the harness.
Now this part I'm not sure about because the design of the silverado vs the sierra is a little different. But on mine once you remove the headlight there will be enough room for you to remove the drl/blinker assembly below it. On my truck there is a white clip you release and just like that the entire thing comes out and you should be good to go.
As far as getting the moisture out someone else may have to touch on this subject. These lights are completely sealed other than were the bulb goes in so I'm not real sure how easy it would be to open the light itself up..or if you even want too do that. Sounds like a heck of a process to get a truck saftey checked up there..good luck!