first and foremost, you're right, your alternator wire is frying for a reason. Follow that wire...all the way to its other end. I think it runs under the crank. you need to check the entire thing and its insulation to make SURE it isn't grounding out. that will cause it to melt. It shouldn't get that hot. THAT IS A HUGE FIRE HAZARD MY FRIEND! When you replace it, make SURE it isn't going to do it again. My other guess would be the voltage regulator on the alternator shat on you but you said it was a new alternator so I doubt its that. Let us know how the new wire does.