Assuming your certain the body earth for the Right lamp unit is good then you have no earth AT the RIGHT REAR LAMP UNIT Stop / Tail Lampholder.
It may just be the connector onto the light unit or the lampholder.
The Right Tail lamp is using the Left Stop / Tail lamp earth, untill you put the brake lamps on, then it looses it.
You should notice the Right tail lamp is dimmer than the left until the brake lamp tries to light up.
Last edited by focusv8; Jan 12, 2019 at 08:27 PM.