Nissan GQ 1992 rear wiring

[nvenn1]

Hi fellas,
helping a mate wiring up his new tray and trailer plug ect. on his freshly cut down extra cab ute. and im having abit of drama with the reverse lights. using the usual volts meter i found the associated wire but it seems to drop voltage occasionaly from 12.5 to 5.4volts

when he bought the car. the wiring was already cut just under the passenger seat about half a meter of loom left. so not sure what else was meant to be back there
any help would be great cheers

[the evil twin]

Most likely a suspect earth (Patrols are notorious for that) you may just have to chase it thru to isolate the faulty piece