Help! I Fried something

This is from another post of mine, but I hope more people will see this.

It looked like my alternator was bad so I ordered new brushes (cause I am low on funds, of course) So I got the brushes today. Put them in, hooked up the alternator, connected the positive cable and... POP-Sizzle! Smoke came off of the wires going into the firewall, I pulled the cable off, black was loose. Now it is dead. What happened? and how can I fix it?

HELP!
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