I have a 2000 S70. Half of my driver side tail light is filled with water. I can't see the way it entered and I don't know how to remove the light. Any advice is greatly appreciated!

 

The inner seal has probably broken if there are no obvious cracks/ holes/ etc, If you pull the lining in the trunk, there should be bolts or wing nuts that hold the light in place and double sided foam tape around it, empty it out, look for the cause and if you cant find it, I would use silicone or maybe gorilla glue all around the seams. I live in Seattle and when I replaced windshields for a living, I would see it all the time, if you cant fix it, watch ebay or craigslist for someone parting out their car.

 

LOL, I have never seen a wing nut on a Volvo but the rest of the advise is good. Another option... you can drill 1 or 2 small holes in the lense, preferably where they are not obvious. There is a break in the top part of the seal somewhere. Do fix or replace it before it causes electrical issues with the light.