I had this problem before. Turns out the drains to the electrical centers were clogged with leaves. When it rained, water actually pooled under the fuse box and by the battery so all my cable s were under water. So I cleaned everything out and the problem went away. Now I regularly vacuum these areas during fall and spring.

If you are experiencing corrosion/moisture at the ignition wire connections, it would be wise to water proof the connections by applying them with the gel that comes with new wires. I forgot what they're called.