When volvo installs genuine accessories in the front of the car they usually link into the back of the cigarette lighter in the centre console.
Along the same vein - do you have a cigarette lighter in the boot of your car? The v/xc70's often have the wiring pre-installed behind the trim, even if that particular model didn't get the lighter as standard.
Check fitting instructions here for more info:
https://accessories.volvocars.com/In...60/2017?query=
Eg
https://accessories.volvocars.com/In...20(-18)%202017