It was the same negative ground wire that had always been on there and had not been removed (it was tight on both the engine mount bolt and on the battery) Same wire gage.
The new one I put on has not overheated (yet) !
The only thing I have been able to think of is that the citrus based solvent I used to clean everything up could have affected the insulation (but, why only that wire and just from engine mount back toward battery ????) :confused2: