Hello
As the tailgate is not metal all the powered components rely on an earth wire in the headlining, which has a habit of failing.
Try running a long wire with crocodile clips from a body earth to the rear wiper spindle which should at least restore the earth path and allow you to open the tailgate.
My own thread on this very subject should help you...
https://www.volvoforums.org.uk/showthread.php?t=305389