take third brake light off, connect it to your battery, if it works then light is not your problem, if it does not work, then its faulty, take it apart and see what you can do with it, if its totally goosed, get another from scrap yard, connect it to a 12volt battery in scrap yard to test it before you pay.
You ask how much from a scrappy, each scrap yard make up there own prices.
There is no legal requirement for a third brake light, so search on line for a cheap one, as there is no rush
Try Renault themselves, you may be surprised how cheap a new one is.
0151 790 0103
474 Woodchurch Road
Edited by Willo_ (2nd Jun 2014 12:27pm)