Assuming that the correct bulbs have been fitted, this will indeed be an earth fault. Extract the bulb holder and study the connections...each bulb outer housing where it is plugged in will be connected together, and this will eventually go into the wiring loom and be connected to the cars bodyshell. Its this wire or connection that is the earth, and it could be worth while using a new length of wire to sort this.
John
August 2009