Nightryder

Hi all, need a little help with an electrical issue, ie Hazard lights
I've fitted drl switch back lights to both front & rear turn signal lights, when I put Hazard switch to on both front lights illuminate(bright light) rears don't, now this is with Ignition off engine not running! (Battery volts showing 12volts. Start engine volts now 14, both front and rear indicators illuminated, but rears are quite dim, I've checked voltage to rear indicator lights at rear main plug & with ignition OFF, im getting max voltage of 7 volts, with engine Running, I'm getting max Voltage of 9 Volts.
Any insight to what would cause this issue will be very much appreciated (MOT Booked on Friday)
Also I've just Replaced the Lucas flasher relay with new 1.
After checking wiring diagram for indicator/Hazard light circuits, I see that this runs through the bcu could this be at fault for votage drop @ rear light unit connection plug(loom side)?