190E Towbar electrics relay and dash light ?

[GoG]

I noticed that my 190E has a relay for trailer lighting and also in the manual it says that there is a green "trailer indicator light" on the dashboard, does anyone know how I would wire these up when wiring up my towbar, I presume if I just tap into the electrics at the rear lights then I wouldn't be availing of these features ?

 

[television]

A hard question,, the lighting diagram is here