I have had problems with my trailer lights for some time and finally today I tried to find out where the problem is. 

 

And I have used my multimeter and found out that the problem must be somewhere in the wiring, the lights are working, the fuse is fine since some lights work. 

 

I only get a faultcode on the left side of the trailer, where do the wires go FROM where I plug in my trailer? 

What electrical kit is fitted?

Don't know, it's factory and the year is 2010 if that is any help. 

So, if it's OE, there will be a control module behind the boot trim on the passenger side. Check fuses 8, 9, 43, 44, 45. The control module can be scanned for faults if you have VCDS.

 

Don't rule out the trailer as the cause either. Make sure all bulbs and wires are ok.

 

I hear you, but as I wrote, I have checked each and every "pin" in the outlet, and only the pins where the corresponding lights are working is giving me 12V when for example I brake or use the turn indicators, so I'm positive that the problem is there. 

You must have written that in invisible ink!

Beware trying to check pin outputs at the socket if it is OE electrics. The connection to them is only activated if it detects that a trailer is connected.

 

Have you checked the 5 fuses I listed above?

Check earthing on the trailer - can cause all sorts of weird issues. Earthing problems very common - especially if trailer has been stored outside.