65 tail light problems

[Hillbilly Deluxe 65]

ok so i ordered all new wiring harnesses for the tail light all the woring from the firewall to the lights is brand new one light works flawlessly the other i just cant seem to get a ground, where or how are the tail light buckets grounded to the truck, i know its a ground problem b/c i can put alligator clips on the bucket and the other end on o bolt or somethin and it works fine

sooo any ideas???