The other posibility is thatthe heath robinson cam device that translates rotational knob movement into a logitudinal cable movement may have come adrift. If it is this, this can be easily rectified by removing teh footwell light panel and refitting the plastic cam in place. I wrote some instructions for this which are in the MGF FAQ which may prove useful.
My guess is that it is the plastic stem, but always worth checking.
My guess is that it is the plastic stem, but always worth checking.