When I have to rewire a trailer plug, I perform a similar test as HB, but use a battery charger instead of a battery. I clip the charger's ground to the frame, and then touch each of the individual wires with the hot lead to test their function. The blue wire is generally used for the brakes, and as was stated, sometimes make a noise when they're activated. When you touch the trailer ground wire, you will get a sharp spark and the meter will jump. These wires are generally the heaviest you'll find in the bundle. The running lights are easy to spot, and with these three wires discovered, it becomes an easy proposition to identify the rest by elimination. The truck's power wire can be usually spotted by turning on the LQ or stall light switch, and when the interior lights illuminate, you have identified another wire. The left and right turn indicators are easy to spot, just use a piece of masking tape to identify each wire as you discover its function. You will have to know the individual functions of your truck's plug wiring, and match your trailer plug to it. In all it's not a bad job, and usually only takes a few minutes. Gard
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