Hello, the TÜV also checks the NSL. Does the NSL have to be switchable even when the light is switched off, or is it enough if the NSL goes on when the light is switched on (of course with extra switch)? D.h. NSL would only be switchable when the light is switched on. The reason is that there is no signal at the NSL socket on the towing vehicle. (Tested with test plug). But I want to save myself the constant request for the error (obvious to the towing vehicle) and simply bring an additional NSL on the trailer. n and when the inspector says “Please turn on fog lamp” with a radio switch, i.e. simply lead the power of the rear light to the NSL with a radio switch. On the towing vehicle, Volvo V70 II, 2001, I never thought there was a signal on the pin of the NSL / AHK removable 13 pole, was retrofitted. However, the NSL on the Volvo works. However, I believe that the trailer, never an NSL was connected … (TEC 490 TSK, 1996)