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This is the cable/connector we need to work with, shown in the green elipse:
Unplug the cable from the back of the TX unit and using a sharp knife (such as an Xacto) and cut a slit about 2 inches long in the cable's outer jacket. Be careful not to cut too deeply to avoid cutting the shield braid which is just below the surface (don't worry if you cut a few strands as that won't affect anything).
Using a pointed object (a small jewler's type screwdriver works well), carefully separate the braid to expose the wires inside. Carefully segregate the yellow wire and place a small piece of electrical type underneath it. This will help in the next step.
Carefully remove just enough of the insulation of the yellow wire so are to be able to tin the wire so we can solder a "tap" wire into it. Then solder a 12" piece of 20 to 26 gauge wire to the yellow wire.
Carefully tape up the exposed wires, taking care not to short the shield braid to the connection you just soldered. Plug the connector back into the back of the TX unit and route the new wire through the opening in the partition used for the fan wires.
Route this wire out the back panel of the DR-1x using the ventilation opening and solder on the supplied Molex pin and insert the pin into the supplied connector body. Finally, connect this to the mating connector coming from the DB15 plastic shell.