V50 3rd brake light
 

My 2008 V50 3rd brake light when brake pedal is pressed works great for view seconds than it starts to flash. Sometimes it works perfectly but sometimes doesn't turn on at all. I have checked the light itself and it works great. So I assume the problem is in wiring but where to look?

Third brake light is 10/19 on the diagram

https://cimg9.ibsrv.net/gimg/volvofo...883ab5612c.jpg

Thanks! But how does it helps me, if I don't know what the rest things in the diagramm are?

It's a standard Volvo diagram - have added some labels
https://cimg9.ibsrv.net/gimg/volvofo...56233cef6b.jpg

Thank you very much!