negative battery wire

Hello all cool site first time poster. I have a 1989 runner 3.0 sr5 4x4 my battery caught on fire under positive terminal under normal driving conditions. I was able to put the fire out but the terminals melted on the wire end. The battery still works the shell is melted a little by pos terminal but tested at 11.8 volts good.I am going to replace battery eventually. I replaced positive cable from starter going to the battery. my fuse box wire going from fuse box to the positive battery terminal melted also i was going to solder or splice that to the new pigtail on the positive battery cable i bought from auto zone. Now the negative cable is frayed at the end i wanted to replace that also being the its 26 years old how do i replace that where does that attach to, From the looks of it the wire goes behind pulleys deep in engine block any one know how to replace that? SRY no pic's Thx any help