Will not start and keeps blowing number eight fuse

Car stopped starting suddenly one day, doesn't make any noise when the key is turned. The number 8 fuse keeps blowing every time I replace it (20 amp). We've already tested the battery, and it's fine. The battery terminals were a bit corroded, one was cleaned and the other was replaced, both were covered in some dielectric grease. After that, the car started twice, but the third time the number 8 fuse blew again, and then immediately once I replaced it.

The number 8 fuse socket tests one side grounded the other not, which is definitely why the fuse is blowing, but beyond that we're pretty stumped.