Dash ilumination is taken off of one side of the sidelight circuit, so it would seem that it is the same one here.
First take out the bulbs front/rear to see if it still blows, as a duff bulb can do it(shorted inside).
If not it is a case of examining the wiring to find a live that has gone to earth, using a metre(resistance) is easy enough though can be time consuming.
tabetha