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might help... I would go to the trailer with a voltmeter, some jumper wires, a new cable that fits the truck, with modern connector and wiring/color conventions, and then test and wire each connection on the trailer individually. First determine which pin on the trailer is ground, using the ohm function of the voltmeter; then connect the negative of a test power source (a spare 12V
battery) to the ground pin and use a 20A fused wire to test each remaining circuit and wire the connector accordingly. Finally, test it with your truck, before going anywhere.