Because no funding was available from any of the studios, François Truffaut and his crew shot the film on the fly on the streets of Paris, often making up the script as they went along. The ending was decided on the basis of who was available at the time of shooting.
As Charlie, Lena and the two kidnappers are driving down the road, a truck in front of them bears a large "Cahiers du Cinema" sign. Director François Truffaut wrote for "Cahiers" and dedicated The 400 Blows (1959), another one of his films, to its founder, André Bazin.
The opening chase scene is so dark because the rain kept blowing the bulbs of the lights cinematographer Raoul Coutard was using to light the scene. Due to budgetary restrictions and the fact he was making a film noir, Truffaut simply decided to continue shooting with minimal lighting, resulting in moments of almost total visual obscurity at points throughout the scene.