Why is it when I turn the ignition key I hear a clicking sound down in fuse box? Replace 30amp fuse and it blew again..

So you are hearing a clicking sound, the fuse keeps blowing and the mechanic says you need a starting harness Hummm?

It sounds like a loose or corroded connection. Check the Battery cables all the way to the starter. Take them off, inspect them and clean or replace if you find a bad one. It could also be a solonoid going out.