I can't imagine what sort of harness problem could cause high voltage. What exactly is the problem? Just the battery light? 15.3 volts is only 4% over 14.7 volts.
There had been a few odd quirks with the wiring in this engine bay. at one point I was getting 14.7@ the alternator but getting 15.6@ the battery, checked all my grounds and added grounds from the battery to the block and block to the chassis on the alternator side of the motor. that's when the the two readings.evened out.