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U Pull has a 56 12V harness for 28.00? Using a stock harness from some ??? Donor will be a nightmare unless you know a lot about wiring, then it would be plain difficult.
First is there something wrong with the current wiring, worn out, messed up, want to add accessories? If all answers are no, then just use what's there 6V wiring is actually heavier than 12 V so will work.
If any answer was yes, then buy an aftermarket harness that have a fuse block, new wire color coded and marked. I use and recommend EZwiring harnesses. Their 12 circuit will do a near stock type truck with a few std accessories for 155.00. Their 21 circuit will handle all the accessories you might want to add for 165.00. Their prices are 1/2 what the big names want and of high quality.