[idlafie]

You're gonna need a FSM along with a voltmeter / continuity tester to troubleshoot this problem. You'll also have to raise the car up on jackstands so you can access the reverse switch on your transmission. Before you go thru all that hassle of raising your car, with the engine off, but the ignition switch in the ACC position & the transmission in Reverse, use the voltmeter to see if you are getting voltage to the wiring harness where the bulbs plug into your kouki tails. If you are getting voltage to your harness then your problem is gonna be with the wiring of the tail lights. If you don't get any voltage to the harness before the tail lights, then it could be your reverse switch on the transmission or a shorted wire leading up to the tails and or burnt fuse in the fuse box. You'll have to trace the line to figure out what the problem is.....Good luck...hope this helped.
ID