You havent swapped out the factory tail light housings for something like Alteeza's or another aftermarket brand have you?
If youve swapped for aftermarket housings swap back, their notorious for issues like this.
If you still have factory housings begin at the tail lights and start tracing all the ground wire, make sure the grounds are clean and making good contact.
Check the bulb sockets and ensure theres no corrosion.
Apply Dialectric grease to all the bulbs/sockets after checking for corrosion.
Check the light harnesses for nicks and cuts that could be causing ground shorts, repair as necessary.