My third brake light which sits in the spoiler has stopped working. Looks like water has got in and started to rust inside.

How do you take them off? Do you need to remove the spoiler or can you pull them out?

Also, where can i buy a replacement for them?

My spoiler is same as this - link

 

Looks like a standard spoiler. The spoiler has to come off to remove the third brake light, it's under a cover on the lower edge of the spoiler, which is held on by two, or three machine screws. The light iself is again held on by machine screws, easy to see once the spoiler is off.

I use masking tape to mark the exact position of the spoiler for replacement, and don't forget some non-setting mastic around all the holes before replacing and tighteningThis stuff is good: http://www.ebay.co.uk/sch/i.html?_fr...All-Categories

You'll probably find that your problem is corroded wiring, or wiring connector.
Best of luck.

If you DO need a new unit, try breakers, e-bay, etc, delaer will be £££££££££.