Back gate to van doesn't close correctly

I have a 1990 Chevy Lumina APV and the back gate does not shut all the way. I get a "Gate Ajar" check light. I can slam the tail gate shut but after driving it around the light comes back on. The gate does not wiggle so it 'seems' shut tight enough. What would I have to replace to keep that closed. And also I had to pull a fuse for the wiper on the tail gate. The wiper would keep moving even if it wasn't on. My front wiper blades have been getting stuck when I turn them on. Electrical problems? :banghead: