Remove one bulb from one tail lamp socket, and see if the others operate correctly. Replace that bulb and remove the next one, check again. Repeat for the other side. If no change is observed when removing a bulb, that bulb is not a problem. It's quite likely that one of your bulbs is not like the others and is putting a different load on the circuit if the ground is not 100%.

If you still can't make headway, remove all the bulbs and insert one bulb. Test again. If that works fine, add another bulb. Test again. Same for the other side. You should be able to find the bad bulb or socket.

Once you find the bad bulb or socket, turn off the lights and examine the contacts in the socket for corrosion, contact with the housing, bulb shell loose, or other issues. The bulb shell on one tail light socket on my '69 likes to pull out of the plastic socket, and when it does invariably it is installed backwards the first try, causing the same symptoms.

Wiring requires patience. Good luck.