Hook up both the black and brown wires to your vehicle's tail lights.

Cobra Trailer wiring:

COLOR CODE: USE:
White Ground
Black / Brown Tail / Side
Red Brake
Yellow Left Turn
Green Right Turn
Blue Aux

I just wired a new car which had the lousy American standard tail lights to work with International standard lighting, as used in the Cobra. Used a Curt 2- to 3-wire converter available from E-Trailer.com. The trailer lighting is now fully functional.
http://www.curtmfg.com/part/56196
Jim