Reverse lights working, Well sort of!!

Hi,
currently learning about wiring issues on the old boy. Managed to update the wiring for the headlights and spots but have a problem with my reversing lights. They work but are very dim. if I take the left side bulb out the right side bulb gets brighter. Have checked the bulbs and the ground wire and they are all good. Any ideas anyone? He failed the inspection (MOT)on them!
Thanks in advance

Replace the Ground wires to the frame.  Run a separate ground for each to the frame……. then make sure that you have 12V on the positive wire, you may need to clean and or replace the female spad connector….