I have just replaced the bottom bulb on my fabia and it seems not to be working too good.

If I turn the lights on it is dimly lit but when I apply the brake it doesn't light up. This is the bulb from the bottom of the cluster.

Any ideas?

Have you put it into the holder the correct way round as twin filament bulbs only fit one way round

+1, if the twin filament bulb is in the wrong way, the light will work the wrong way round. Lower output is because the power for tail (5W) is being sent to the stop (21W). Turn it round.

They are in correctly. If I put them in the opposite way they just use the 21w. To be sure. The dim one for night time when the lights are on is the higher up part inside the bulb?

Is the Bulb perfectly alright? Try to connect it to an external Source to confirm. Also, the contacts might be worn, try cleaning them with a contact cleaner or scrape them with a blade and try.

Is the Bulb perfectly alright? Try to connect it to an external Source to confirm. Also, the contacts might be worn, try cleaning them with a contact cleaner or scrape them with a blade and try.

If you don't have an external source you could always try the bulb in the other side and vice versa.