I just did this in my refresh March 2017 S90D and found a slightly different configuration in this location. I figured I'd post some pictures in case anyone else was looking for a 12V source in the passenger foot well / glove compartment.
You need to pull most of the side panel of the center console off ( careful with the rear, the vents snap into this piece and need to come out first), I was never able to remove the center console passenger side piece (not enough leverage), but got it off enough to pull the carpet piece under the glove box out. Here is the back of that carpet panel
And behind it you will find the USB connectors (they were labelled such in my car, something like USB to MCU or something), tucked back into the center console is a connector that you can unplug by pushing down on the top of the white tab and pulling out, this is where my cigarette lighter and rear usb ports seem to get their power from. I just picked a red wire and black wire on the outside, attached a multimeter, locked / unlocked doors and watched power go off and on.
Matt