Those are actually stock on any truck over 80" wide and is factory wired. I had an '87 C30 CC that was set up that way and so is my '85 K30. No one is making the tailgate with the holes drilled out for the lights. You will have to do it yourself. There are 2 screws used to hold the housing on and I believe a 1" hole in the middle of each light for the harness to fit through the gate (I haven't had mine off yet). If you need measurements, I can remove the lights and take some pictures/measurements for you. There should be a 2 wire plug for the harness near the lower left tailgate hinge under the bed. I'm going with LED clearance lights on the gate. Here's the part numbers I'm using:
Lights: Grote #G1502 (3 LEDs)
Housings: Maxxima #M20326 - Black
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