The extension harness is part number 84376073. it plugs into the back of the USB block on the bottom of the center stack beneath the buttons for traction control, hazards, etc. There should be an open port for it already. this then plugs into the main harness from under the front passenger seat. The cable should be located under the cup holder/tray.
BUT... I just completed this swap also. My 2020 Silverado TB LT had the jump seat/bench seat and I switched it to the full center console without the wireless charger option. In addition to the extension harness, I purchased the new wire harness that plugs in under the front passenger seat and is responsible for powering the anti-theft module (mounted under the cupholders), the light in the bin, the antenna for the key fobs and/or buttons on the handles, and the rear USB ports. With all things plugged in, including the extension harness, the USB block in the bin gets power and charges phones as intended, however the radio screen/infotainment screen stays stuck on the Chevy bowtie screen. With the USB block unplugged, the infotainment center works normal.
Very frustrating but I hope there is a remedy for this issue. I reached out to some friends at a GM dealer in service and they seem to think that GM hasnt released the programming necessary to make the truck accept the newly installed USB block since it was not originally equipped with it.