On my C4 (RHD) it is in the passenger footwell. Remove the plastic cover beneath the glovebox. You'll find the switch behind this.
Disconnect the connecting plug and twist the switch a quarter turn anti clockwise and pull.
To fit the new one its the same process but in reverse.
Its a really quick and easy job. Switch only costs bout £6 from Citroen dealership. Common problem.
CRT
February 2010