Thanks for all the hints. I will look into the gap seal tap - it might work. The truck is a 2500HD DA CC 4WD and it has a rounded over tailgate, so the flap on the topper seals great. From inside the topper, looking out, I can only see sunlight along the vertical edges of the tailgate/truck bed so that is why I am assuming the water leak is there.
My truck primarily tows a gooseneck horse trailer, but we have the topper so we can run errands in the rain and pick up bulkier stuff then the tonneau cover allowed us to. And the Velcro on the tonneau was pretty much shot, so it wouldn't stay attached anymore, and needed replaced.
Since putting the topper on the GMC, it has proved priceless for "hotel" style family trips, as there are places we go to visit family where pulling a 43' toyhauler is just not feasible. No sense having to park the TH at a campground 30 miles away when the GMC will fit perfectly in the driveway while we sleep in an extra bedroom... BUT! It really sucks when rainwater soaks EVERYTHING in the bed of the truck. There has literally been water sloshing from front to back in the bed, it leaks that much. Not fun.
