Depends on how much time and money you have. We had a similar seam issue, there were gaps. We removed the trim, we used the seam tape mentioned above to push in the corners, tape it up, then put liquid sealant in the trim and used new screws to replace the original trim. We also resealed the edges of the trim. Its not exactly beautiful but works great. You may want to spray in some insulation before sealing it up. On the tape, it works great but IS hard to remove if you need to, and use it on a clean surface.