Trailer Plug Wiring - Help

I have discovered the wiring on my patrol to the trailer plug is dodgey and i need some help with a couple of wires. The functions are same as a normal trailer plus it has electric breaks and a wire that charges the battey when the car is running (i am pretty sure that this is what the anderson plug does, which i think is ok.

i have the blinkers, tail lights and break lights working on the trailer but still have three wires left

These are
1xthin Brown which lights up the test light when the breaks are on (seems to do exactly the same as the yellow i have conected for brake lights)
1x thin black which appears to do nothing, and does not act a an earth as i have tried this. this one is a real mystery
1xthicker black which also comes on when the brakes are activated, it comes out of the back of the anderson plug. my guess is that this is the electric brakes and i think i know where it goes. but does the thin brown also go with it in the plug or with the yellow?

OK i hope this makes enough sense for someone to be able to help me . cheers dave