The assembly includes a front face, a bumper skin, two flexible brackets, and two light units, each of which is rigidly secured to a corresponding bracket. The assembly further includes pre-positioning and guide elements adapted to provide pre-positioning that allows a first end of each of the brackets to move in a substantially axial direction relative to the front face, the elements including members for driving the first end in axial translation relative to the front face, the members operating by meshing, and elements for releasably receiving an adjustment tool in a meshing position. An adjustment tool, and a method of assembling and mounting such a front unit assembly are also disclosed.