Several cars develop this problem. It's usually due to a too small connector in one of the plugs in the rear part of the ceiling, where the tailgate harness connects to the car.
Typical symptoms are
- Tailgate signals open, whilst being closed.
- Tailgate will not open.
- Rear wiper doesn't work.
- Rear demist doesn't work.
In this case, you have to open the tailgate to fix the problem. There is a mechanical emergency opening possibility, but it's quite difficult to reach.
When our 2016 also developed this issue (the 2012 I repaired several years ago), I found out an easier way to open the tailgate.
- Attach an electric wire to ground somewhere in the car. I used the lock buckle in the car's body at the rear side door.
- Remove the plastic cover over the rear wiper drive shaft. There's a small hook on the inside. Push that to the side, then push the cover back out.
- Connect the other end of the wire to the drive shaft.
- Try open the tailgate.
In my case this worked immediately. Accessing the connector above the rear panel in the ceiling is then easy.
At the local Volvo workshop, I learned that their method is to have one person crawl into the car. He's then hammering on the ceiling with his fist, while the other guy tries to open the tailgate.
Sometimes that works.