I got a 94 Chevy silveraldo 1500 extended cab , i have had the truck for 10 months and 2 weeks ago i noticed a few drips leaking inside on the rear window towards the pass. side,on the outside it has like a black weatherstrip seal i tried to sillycone it and it still leaks. i have 2 questions.

1. how can i tell if someone tampered with the seal to cause it to leak?

2. what can i put to seal the leak up?