Does copper wire go bad?

Update:
Even with the motor idling I only read 10 volts at the rear side marker lights. Battery voltage is 12.4 (motor off) 12.8 (motor idling), and 14.1 (fast idle). The voltage at the rear side marker lights goes up to over 12 volts at fast idle. The turn signals flash at the current rate but very weakly at idle and at fast idle that brighten up quite a bit. Am I due for a complete wiring harness upgrade or could it be a charging systim glitch? Anyone else have similar symptoms and found the correct fix? Thank you.