Yes it should be a dim tail light and a bright brake light.
Check for power on the wires, you may be able to just swap around the bullet connections and use the unit as tester. There might be no ground wire, OEM lights did have it but aftermarket ones don't always. This should be a green wire.
The brown wire - assuming a stock harness - is tail light power. Should be +12V when the key is on and the lighting switch is on (if you have that switch, on with key if not).
Green/Yellow is the brake light power, switched by key plus brake pedal/lever switches.
Does the tail light come on with key in "P" (park)? If it does, check your TAIL fuse. If that's OK maybe the ignition keyswitch or its connector is wonky.