Can anybody tell me what causes clear corners to fog up and how to fix them so they don't? I bought a set about two weeks ago and one of them is fogging up now.

 

were the yellow markers in the corners taken out? they need to be pulled out, let the moisture dry up, them reseal the edges

 

Condensation. Dry them out and run a bead of silicone around the edge.

 

I know condensation is the end result, but didn't know why there were condensing. Wasn't sure if they were suppose to be sealed up or not. I had a guy tell me to drill a hole on the bottom to let in air so they wouldn't fog up. That didn't make much sense to me but figured I would post and see what some folks out there thought.

 

That helps too.

 

The Denali projectors actually have a hole behind the reflector that goes to a rubber hose. The Sierra headlamps don't.
If your corner markers are getting condensed(I'm assuming they're clear corners since the same happened to mine, but not my stockers) get some wetsand paper and lightly sand them, that should take the yellow crap off. Then add some more silicone to seal em up well.

 

when i installed my PIAA's (foglights), they fogged up the 1st week. there was a hole in the back plate, but it was on top. so i removed the back plate and rotated it so the hole is toward the bottom. haven't seen any condensation yet

 

had same prob with mine, i took them out and filled with water to find leak then i could seal it up. shake out as much of water as possible then blow dry with compressed air. i poured some rubbing alcohol in them to evaproate trace water that was left, make sure the air compressor has had condensation drained from tank recently or has a water seperator on it like you would use to paint with

 

for the yellowing, you can use swirl mark remover, and Meguairs makes some clear plastic polish and cleaner. the polish works GREAT, makes them look good!

 

Had the Same problem first time i waited to long and the water ate all the crome out of the light i did what Nick99 Said with the silicone Around the Seem and its never happend again and its been about a year or longer........