It is hard to say without testing the fixture, so lets test the fixture.
2 filament bulbs have both the parking light and brake light in them. What you want to do is with a 12 volt
battery test each wire to the + terminal of the battery while the fixture is attached to the - terminal of the battery. The brighter light will be the brake light and the dimmer will be the parking light. Now that you know what the fixture is set up with you can test the trailer wiring, with a test light or meter, to see which wire is brake and which is parking light.
The fixture is grounded through the trailer body when installed.