Turn and Brake light on TT only work on one side

The OP said the tail lights work but turn and brake don't. Turn and brake are the same thing (same filament). It may simply be a burned out bulb. Change the bulb or connect the trailer wiring to another vehicle to see if it works.

If a new bulb still doesn't work even with a different vehicle, the next place I would check is the trailer plug connections. Pull the housing back and make sure all the wires are still connected and tight.