The extinguisher should be visible and easy to reach. Mount the bracket on a strong, stable surface, sloping, vertical or horizontal position. The most common places to mount a fire extinguisher are in front of and behind the seat.

The reason for placing it on the bottom of co-passenger seat because of easy access during fire, when person is cramped/stuck inside the car in the major accident or the whole steering column is collapsed over him

mounting on a trunk of the car is not recommended