The 2008 STX should have a set of fuses behind a pull-away panel on the left side of the glove box.
In the IPDM/ER box in the engine bay you should find a pair of 10A fuses in the middle. Remove the cover, look at it so that the 4 socket/relay line is on the left, the 5 socket/relay line is on the right, and the line of fuses in between is what you're looking for, these fuses are - counting from the "top" - numbers 5 and 6.
Inside the cabin, check the 5th fuse spot from the bottom in the left column. There might not be fuses in all of them as the bottom two are spares.
If that doesn't work, check that there's an earth back there. Sometimes, if the stereo system has been played with, the earth to the instrument cluster can waver and this affects the tail lights. Not sure why, but it happens.