On most cars, it's just fed from the side light circuit(s). If both tail side lights are working, then I doubt it's a fuse.
Pop out the holder again and test the voltages to both contacts with respect to a good earth (e.g. the bolts on the tailgate striker.) Do this with a good bulb in place and the lights turned on. You'll get one of the following results.
0v on one wire, 12v on the the other. Either the bulb is faulty or there's an internal fault in the bulb holder. This could be one of your corroded contacts
12v on both wires. There's a bad earth to the number plate lamp. You'll need to repair this, though it's often easier just to run a fresh earth wire.
0v on both wires. You have a poor connection to the source light cluster. Again, you can try to trace and repair this but it's often easier just to run a fresh wire.
Currently in Horatio's garage:
2010 Laguna III Initiale.