Problem with trailer lights on 2014 Yaris

[RWey52]

I have a 2014 3-door hatchback. Installed a hitch, and then bought a 5 to 4 converter box at O-Reilly's. Upon inspecting the tail light wiring under the removable covers, there's two wires going to each bulb. One switched hot ( +12 when called for ), and the other is a white/black tracer. I concluded the white/black is the common for each bulb, but this wire does not ohm out to the chassis ground. They do however show 0 ohms between two different wht/blk wires. I'm thinking they probably run a looped circuit for each bulb for diagnostic and burnt out bulb detection ???

Anyway, I hooked up the four signals properly ( running lights, brakes, left and right signal ), then I chose the white/black wire from the brake bulb for the ground.

Before I go further, the trailer IS wired correctly and IS functional. When plugged in, however, the only lights that work on the trailer are the running lights, which flash when the signal is on. No break lights.

Is this a problem with the discrete ground wires going to each bulb? Am I mistaken assuming all white/blk wires are a common ground ?

Thanks for help.