Poons I would say if you have proved that a) an individual button assembly works in another position or another works(or the fault stays the same) in the same position as the faulty button then that is ok and b) that you have proved that the car module is ok by replacing it then its most likely that the main controller interface is where the problem lies. This is probably either a software issue(for some reason has turned of the level or even put a secured link on it) or that a hardware issue has happened( such as a opto coupler or the like). Either way I would get another board to at least prove this.