V60 Dual-Zone to Tri-Zone Climate Control: Adding Rear Seat Air Vents?

[IMAY2023]

Why don't you wire the 3 temperature sensors, and install them in the air vents under the front seats (2 in left vent and 1 in the right), and you can activate the 4 zone climate?

First of all, there are four of them and not three.  One under each front seat (L&R)  and 2 for the central console rear vents (L&R) . 
The problem is each sensor goes directly to the climate control unit which is located on the HVAC blower behind the dashboard, via car's two main body harness connector. 
So, it's not possible to wire the four sensors without actually pulling out the dashboard.