Recently, my left rear fog light is on when the rear lights are on, and I can't turn it off with the fog light switch.

Also, the trunk cover release button (on the trunk cover) sometimes doesn't work. But when I push the button while the rear lights are on the left fog light turns off (as long as the button is pushed).

Any idea where to start looking for the cause of this behaviour?

 

Check the boot loom at the hinge point, peel back the rubber boot & check that the wires are not broken.

https://i.ibb.co/F5RGr1F/X250-Boot-Loom-Issues-v3-1.jpg