2002 W900L load light/cargo light wiring

Hello and thank you in advance! As stated above, 2002 W900L daycare, has the factory switch to the right of the ignition switch that appears to be for cargo/load lights. However, day cab haul truck that has a headache rack on it that I am adding lighting to. Any suggestions as to where exactly I might find the pigtail/harness that the switch in question will control/activate? Anyone know exactly how said wiring should be labeled? I have looked under hood and frame rail, obviously to no avail. Just don't want to go thru the trouble of running wires, then someone smarter than me says "goofy ###, why didn't you just grab power from such and such marked wire that is located right there". Lol. My apologies for the lengthy post and again, thank you for any and all help in advance!