Philippe,
This design does not have a regular socket for the tail light bulb. In your picture the copper strip is the power lead which should rest against the back end of the bulb. The left light has a hole, lined by a rubber grommet and the bulb is press fit between the grommet and a grounded L shaped piece that contacts the base. Then this copper strip presses against the bulb contact at the base to complete the circuit. If there is no socket or other device to hold the bulb, you need to return it to the vendor. The other alternative is modify the light to accept a two filament bulb but you shouldn't have to do that. Maybe this is just a one off dud, that missed a step in manufacture. If not, this is sad.
Kent
