My two wheel drive turbo diesel, blows a directional fuse (intermittently) when I back up.

This used to be an occasional problem during wet weather. Now if I reverse for more than two/three seconds the directional fuse blows (i upped to a 30amp from 20amp to get the slow blow). I eliminated directional signal & multi-function rear-view mirror as a cause. Only back-up circuit causes the problem. I fully inspected the harnesses and can see no problem. Must be more trucks out there doing this. Any ideas for further trouble shooting?