Need help trouble shooting a short
 

I'm tring to figure out a short to the starter on my sons 1995 Cadillac Deville. I have a good battery and a new starter, when my son put in the new starter he connecter both the positive wire and the wire to the neutral safety switch to the same terminal.I pulled the starter back out, had it tested and put it back in correctly.if you try to crank it it blows the fifty amp starter fuse immideatly. If you jumper it from the ignition post on the starter directly to the battery while someone turns the key, it cranks over but wont start. Any thoughts? Thanks in advance!