starter fuse blows when i turn defroster on????

OK so i have been having a problem with my starter fuse blowing. I replaced the starter thinking that was more than likely it because it is like 13 years old. but it still blows, the wiring all seems good. but if i put in a brand new fuse, turn the key to on but not start it, and flip the rear defrost switch it blows the starter/rear window defrost fuse? I guess it could be a number of things but does know what it could be? (ignition switch maybe but i dont know how to test it):dunno:
thanks for any help
it a 97 auto 4.0

The rear defroster runs off the starter circuit. It could be the defroster switch, relay or the embedded defroster. Try testing the switch and relay and then the wiring for a short.