I found in my car the previous owner coated and painted a lot of stuff. So the grounds were poor for the tail lights, since they go through the body.
They make new pigtails for the lights with an extra tab on them for a ground wire.
I did that and it eliminated the ground issues.

but as said above you might want to test that out.
add an extra wire around the socket where it "holds" on to the tail light housing and give it a solid ground see if the dim light comes up to the other side.