Hi, remove the switch & you should immediately see the back of the bulb holder on the side of said unit..
The holder could just be making bad contact on the PCB, just remove the bulb holder (a bit fiddly) very gently prise the wire that comes out of the bulb & wraps round the holder out of it's groove slightly with a thin blade implement so it's more inclined to make the contact & refit.
Of course, if that don't work the bulb could be duff.
Also while the switch is out (& connected up) check the PCB contact with the bulb out with a multimeter while the side lights are on.. (just to make sure the connections OK on the loom.)
