IE: check fuses, check that the wires are supplying power to and from the switch.I would look at the switch.
IF, I repeat IF you find a blown fuse you MAY(no guarantee) have a bad bulb. look for one that is damaged.
Again IF the fuse is blown you could take out all the bulbs and see if the fuse does not blow.
you have one for sale? Or can point me to the correct replacement?