after several years in the weather, the vinyl hardens and cracks, probably that's what happened to yours. I can always find a roll of it at my local dealer. I'm now replacing all the vinyl on my '05 Trail Cruiser, top rail and corner trim. Whenever I do that, I get new whitehead, square drive screws, coat them with caulk and replace each (usually) rusted screw with them.
Once the vinyl starts crackin and breaking I think water can follow the screw into the trailer.
1994 GMC Suburban K1500
2005 Trail Cruiser TC26QBC
1965 CHEVY VAN, 292 "Big Block 6" (will still tow)
2008 HHR
L(Larry)V(Vicki)J(Jennifer)J(Jesse)J(Jason)