On a real computer now so can wax a little more lyrical with a decent keyboard....
Yes, Nos- spot on - it's the "extras" socket with the white lid here in the UK. "secondary" power for things like reversing lights, caravan battery / fridge etc.
Random wiring diagram grab from google -
Tow Bar Electrics
If you have a 13 pin socket on your car, take the charging cable from pins 10 (+ve split charge) & 13 (earth for pin 10 etc)