The brown wire should be live at the back of the car when the light switch is turned on bringing power to the tail lights and license plate light. Look for the 20 Amp fuse marked "tail lamp" in the fuse box see if it is good it brings power (orange wire) to the light switch. After these two tests a wiring diagram is needed to do continuity tests on the light switch to see if it has failed.
I had the same problem and tracked it down to replacing the headlight/turn signal stalk on the steering column. I found one for $30.00 + s&h on ebay. Not too difficult(got away with not pulling steering wheel by angling hex screws in.) New one was touchy in the same way, but could make it work by twisting the switch on the end of the stalk.