How do you find out why a fuse keeps blowing out?

First check what's actually serviced by that fuse, maybe you've added something that wasn't there from the beginning, pushing the draw over the allowable limit.

If that still looks OK, start by disconnecting the stuff hooked up to that fuse one by one, see if that can narrow things down.

If that doesn't show anything, then you have to start inspecting the wiring itself for damages.