Wet passenger floorboard

Now that duck season is over I was cleaning out the Burb. I removed the passenger side floormat and it was soaked with water. I have Catch All floor mats so even if wet boots were in the truck the mats would catch all of the moisture. It is not antifreeze judging by the lack of odor and color of the paper towel I used. I know there is a tendency for the HVAC drain to clog. Where is this drain and what is the best way to clean it out?