Easiest way to check this is......
Digital Multi Meter with a Tone for continuity.
Place one probe in the fuse holder for which ever one is blowing. (make sure touch the correct side of the fuse holder/ the side going out to the source, NOT the supply side) If your not sure, turn the key on and use your meter to see which side you have the 12VDC on)
Place the other probe on a ground.
Start twisting, tugging pulling your wire harness from the fuse panel back or from the tail light up till you hear the tone go off from your meter.