I read an article on this a while back. Take the light out and take the lense off. Wipe the condensation off the lense. To stop it happening again drill a small hole on the back of the light. Haven't done it myself but i've got the same problem.
i had my entire front bar off this weekend cause i was "playing around" and found my fog lights were FULL of water... ooops...
drained them out but have stuck screws so didn't get a chance to do anything else to them. i did notice on close inspection there is a built in vent so drilling a hole isn't needed, i would probably recommend checking to see if the vent is clogged up.