easy one. Two options. If you use either one, you should first clean the car as best as possible.
for light smoke odor you can use an Odor Bomb. Just do a search on ebay for Odor Bomb and you will find an assortment of products. Professional Water/Fire Restoration companies use a Fogging machine which is pretty similar to this. It uses an odor counteractant to knock down the fire smell so that they can do additional work in the structure.
for heavy smoke odor, buy a small Ozone Machine. Just do a search on ebay for Ozone Machine or Ozone Generator. They are $50. Just put in the car, close the windows and run it for 2 to 4 hours. Fire Restoration companies use larger units to remove the remaining odors after all the work is finished.