ACR MECH WORKING ON APU FEED LINE; ACCIDENTALLY REVERSED TERMINALS. LIGHTING CONDITIONS WERE POOR AND THE TERMINALS 6NO AND 6NC; WHICH WERE CROSSED; COULD BE EASILY MIXED; AND WERE. WHEN; AFTER 36 HRS OF FLYING; GND PWR WAS CONNECTED TO THE ACFT; EXTERNAL PWR WAS CONNECTED WITH THE SWITCH IN THE OFF POS.

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ACFT WAS FOUND TO HAVE APU GENERATOR FEED LINE BROKEN BTWN THE APU AND THE APU GENERATOR CONTACTOR. I REMOVED SEVERAL TERMINALS; ENSURING THEY WERE MARKED; ON THE CONTACTOR TO PREVENT SNAGGING OF TERMINAL WIRES WHILE REPLACING THE FEED LINE. AFTER SPLICING OF THE FEED LINE WAS ACCOMPLISHED; I RESECURED THE TERMINALS TO THE CONTACTOR. THE APU WAS STARTED AND THE SYS CHKED; AND THE ACFT WAS DISPATCHED. OVER THE NEXT APPROX 36 HRS; EXTERNAL PWR WAS NEVER APPLIED TO THE ACFT. WHEN IT FINALLY WAS; THE ACFT WAS FOUND TO ACCEPT EXTERNAL PWR WITH THE SWITCH IN THE OFF POS. AFTER TROUBLESHOOTING; MAINT PERSONNEL DISCOVERED THAT TERMINALS 6NO AND 6NC WERE CROSSED. THE WIRES WERE UNCROSSED AND THE PROB WAS RESOLVED AND THE DISCREPANCY WAS CLRED. WORKING IN THE POOR LIGHTING OF THE ELECTRICS AND AVIONICS COMPARTMENT AND MY FLASHLIGHT DIMMING FROM CONTINUED USE; I EVIDENTLY MISREAD THE WIRE MARKINGS 6NO AND 6NC AND CROSSED THEM AS I RESECURED THE TERMINAL WIRES. THE IDENTING MARKS IMPRINTED ON THE CONTACTOR WERE NOT READILY VISIBLE TO ME; BEING OBSCURED BY THE BODY OF THE CONTACTOR.