There is a known tailgate earth wire fault for which Volvo have issued a technical sheet but this YouTube video from VolvoSweden shows exactly how to fix it. The fault can cause all manner of problems for tailgate mounted equipment and even if it turns out not to be the cause of your problem, you won't have wasted your time replacing the faulty connection as it is a worthwhile preventative measure.
https://www.youtube.com/watch?v=PRIUBEqEgAA