When I bought my car, the driver side clear sidemarker had a bunch of water in it. They're cheap enough so I just ordered replacements. Turns out the water in the old one had actually started to corrode the inside of the light.

Now, only a couple of days after installing the new lights they're all fogged up too. What gives? Now I'm concerned the fog will turn into droplets again.

Anyone else have this? Is there any fix?

 

That seems strage that a new one would fog up also. Have you checked inside your wheel well to make sure the small plastic plug that is removed to release the spring on sidemarker when changing it out, is still there. If it is missing, you could be getting water into the sidemarker when driven in the rain.