Running lights work turn lights and brake lights do not on 2004 navigator-everything works fine on my sons 98 suburban trailer brand new

I would try hooking the trailer up to another vehicle and see if everything works right if everything works on the trailer I would start testing the trailer plug on your truck could be a wire has broken.

Headlights must be on for the interior trailer light to work.

suggest downloading the trailer pin standard for whatever number of pins your trailer plug has and tracing the wiring from the colors . The standard will tell you what color goes to what light and what pin number it should be on . Also 5 pin and 7 pin are different . You can plug a 5 pin into a 7 pin but the wiring is different .

Also check the trailer socket is grounded at the car body properly. As this can cause lots of weird symptoms.

There is a fase under the hood marked trailer tail lights,it is a 15amp fuse and if continues to blow ther is a problem in the trailer wireing or the plug on the trailer.

check you bulb and there sockets its possible that you burned out one of the two

Have you tried a heavy duty flasher? Also, if your signals & brake lights are 2 different bulbs, as in amber for signals & red for stop, then you will need a small in line converter to separate those 2 as most vehicles use same bulb for signal & stop.

If you have amber signal lights & red brake lights, it's 2 separate circuits. You'll need to buy a small converter that you wire into harness to correct that. Not expensive, try autoparts store or RV/trailer sales, should have them.

It sounds like your have may have your brake light and running lights crossed on your wiring harness to the trailer. Good luck and hope this help's

Are you running the "flat 4" connection or round 7way ?

Assuming you don't have a trailer connector the bulb failure warning module might be you problem