I have a 2003 G35, & the running tail lights wont come on. I've changed fuses, light housings, bulbs & turn signal switch. I heard of the brake switch by the pedal, but my brakes & cruise control works just fine

 

Use you multimeter and check for a possible missing ground. Did you test the bulbs?
Also the problem could be in your wiring harness?
Follow the schematic diagram in the FSM
Good luck

 

Only way to find this issue is with a meter or test light, good luck, pull a bulb and get a test light/meter in there and see if its even getting voltage, if its both taillights more than likely its something upfront like a fuse or bad ground/wire somewhere, i highly doubt both light decided to take a vacation at the same time and just stop working on you

 

Probably broken wires in the trunk harness, ring things out with a multimeter.