None. No ground means no circuit.
Check all your fuses, you may have a fuse solely for the trailer lights. Check all your wire connections and if you used wire nuts, replace them with silicon filled wire nuts. They are normally used for low voltage lighting and irrigation. Check for continuity in the wiring harness. Test the lights directly with a 12v power supply.
That's all I can think of now.
Thank you, I'll try all of that before work tomorrow. That first thing you said though, how could that be true? The lights worked fine for the first week and the car-side ground was just floating around in my trunk, disconnected.
The tail lights do have individual grounding wires and I was unsure about how to secure them. They are mounted out on a 4x4 piece of wood on a small piece of aluminum. I originally had them grounded to the aluminum bracket via the light's own mounting hardware, but maybe I need a more substantial ground, like a wire leading all the way back to the metal trailer frame?