my mate had on on a renault megane
it was a thin slice of plastic a bit like shim with double sided sticky tape on one side. but it went on the inside of the light not on the outside so it couldnt be seen during the day.
the light cover/reflector (the red bit) still looked the same during the day but when you put the brake on you got the word megane show through.
looked ok but the rest of his car was a dump but he loved it.
i dont know if these go on the inside or the outside though, looks like its the outside by the picture so im not sure what the final thing would look like. could be good but it might look propper gash when its on, and for 9quid plus 5quid postage thats a hell of a lot to pay for a sticker you could probably cut out of a sheet of black vinyl for 99p is it really worth it.
but if they came up with some good quotes it could be fun.