1994 Jaguar XJ6 Tail light&marker lights

my problem is i have no tail lights or marker lights .i have checked all the fuses ,traced all the wires down,changed all the bulbs,cleaned all the contacts,grounds and still nothing! HELP could it be the light switch?,relay? i do have all my turn signals,brake lights,emg: flashers and headlights. could i bypass the factory switch and wire in a aftermarket one? Thanks