Additional brake light of a motor vehicle for being arranged in the area of the rear of the vehicle, preferably in the area of the rear glass of the vehicle, wherein an additional brake light is formed as a thick-wall light guide with at least one optical element (5, 6) arranged thereon and comprising at least one light source (7, 8), wherein the light guide comprises an entry surface (4.1), which when mounted in the vehicle at least approximately faces a roof of the vehicle, and an exit surface (4.2), which at least approximately faces the direction opposite to the driving direction of the vehicle, wherein the light guide comprises two legs (4.3, 4.4), wherein at least one of the legs (4.3, 4.4) comprises at least one flat side, wherein the optical element is provided on the at least one flat side of at least one of the legs (4.3, 4.4) of the light guide, and is formed as a holographic film, characterised in that - the light guide is formed in a cross-section as a dual leg element in the sense of the letter L, wherein a first leg thereof when mounted in the vehicle is a vertical leg (4.3) and located approximately in the plane of said rear glass of the vehicle, and wherein a second leg thereof when mounted in the vehicle is a horizontal leg (4.4) which is associated with a second end of the first leg (4.3) and located approximately horizontally, respectively, approximately in parallel with the plane of the road surface the vehicle drives over, and wherein said first leg (4.3) is formed at the first end thereof with an entry surface (4.1), and said second leg (4.4) is formed at the free end thereof with an exit surface (4.2), - a first light source is arranged in the immediate vicinity of said entry surface (4.1) of the light guide, so that the light beam from the light source enters through the entry surface (4.1) into the light guide and afterwards, travels along the legs (4.3, 4.4) pursuant to the total internal reflection, and exits through the exit surface (4.2) performing a primary optical function of the brake light, - said optical element provided on at least one flat side of at least one of the legs (4.3, 4.4) of the light guide can be lit by means of a second light source to perform a second optical function of the brake light. - the light guide is formed in a cross-section as a dual leg element in the sense of the letter L, wherein a first leg thereof when mounted in the vehicle is a vertical leg (4.3) and located approximately in the plane of said rear glass of the vehicle, and wherein a second leg thereof when mounted in the vehicle is a horizontal leg (4.4) which is associated with a second end of the first leg (4.3) and located approximately horizontally, respectively, approximately in parallel with the plane of the road surface the vehicle drives over, and wherein said first leg (4.3) is formed at the first end thereof with an entry surface (4.1), and said second leg (4.4) is formed at the free end thereof with an exit surface (4.2), - a first light source is arranged in the immediate vicinity of said entry surface (4.1) of the light guide, so that the light beam from the light source enters through the entry surface (4.1) into the light guide and afterwards, travels along the legs (4.3, 4.4) pursuant to the total internal reflection, and exits through the exit surface (4.2) performing a primary optical function of the brake light, - said optical element provided on at least one flat side of at least one of the legs (4.3, 4.4) of the light guide can be lit by means of a second light source to perform a second optical function of the brake light.