Tito, I would start with plugging something into your trailer plug and seeing if that item flashes. If it does, then the problem is inside of the tail light and I would start with replacing the bulb, then next going to the plug for the entire tail light housing and making sure that is in good condition (ie, all terminals are a nice silver color and not a brownish color, clean if necessary). From there, you would need to use a multimeter to see if you are getting power to the plug. IF you are, the problem is inside the tail light and odds are, you have to replace the tail light.
If you connect something up to your trailer plug and that don't light, then I would start with replacing your turn signal stalk. This is the weak link in the system and they are known for failing. From there, you will need to do some probing with a multimeter to see where the power is and where it is not.