My colleague just passed the inspection with the fire extinguisher fixed in the trunk of his car (which is a 4x4). In my car (which is also a 4x4) the extinguisher is installed in the back also. I don't think they would have installed it there if it would not pass technical inspection at that location.

The important thing is, the extinguisher must be strapped down using a proper mounting.

I have rented a sedan a few weeks ago and the fire extinguisher installed in the trunk. I don't think it is a problem.

If in doubt, call the technical inspection agency.

Regarding the use of fire extinguisher AFTER an accident, if the accident is severe enough to collapse the steering column (which is severe enough to activate airbags also), odds are the driver is - at least - partially incapacitated (incoherent or unconscious) and probably won't be able to use a fire extinguisher regardless of where it is located.