There's no magic fix for cigarette burns - as a smoker of 47 years I know this very well. Vehicle carpets are usually man made fibre (fiber) and the heat melts, disfigures and discolours (discolors).
You might spend a fortune and have a specialist re-carpet the interior or you might try patching - i.e. cut out the offending burn and glue in (or use very sticky carpet tape) a piece of good carpet from an unseen area like under a seat for instance. If you take care the join may only be visible to somebody actually looking for it - unseen by a casual observer.
Good luck...
Peccavi
July 2009