Steve
First try spraying WD40 on the lock where you can get at it.  Try tapping
the offending door around the lock area and then try to unlock again. If all
else fails remove the door cover from the inside and then you need to brake
the lock and replace with a new one. I have had the same problem with a 406
on both rear doors. The problem has 2 causes, first the lock is entirely
plastic and second the deadlock is all plastic and wears easily. Remedy to
increase lock life is not to use the deadlock atall.
Mike