According to the NAS 90 diagram, the slate/black wire comes out of the ECU as a command signal to turn on the condenser fan, labeled as relay 182, pin 4, by grounding it. The power for that relay is on pin 2 of the relay and via a bunch of splices, eventually getting power from fuse 7. Fuse 7 also is your main ECU power, so if it blows you are hosed.
As for the melting, I would say that the relay shorted somehow.... the ECU commanded the fan to turn on, but the relay was shorted inside, and it melted the wire. The fuse is big enough (20 amp) that it didn't blow in time. That's a SWAG.
Fuse 17 seems like a lot of unrelated stuff... rear window wash/wipe, brake lamps, hazards, but DOES have the compressor clutch relay, A/C blower relay, and even worse the engine immobilizer ECU!