Phil.
Yeah that stinks!
Since you di dnot tell us where the spill was I will give you a general remedy for just about all areas.
First do NOT use an extractor to remove any part of the spill, even after it has dried, because the fumes may still be flammable.
Go buy several boxes of Baking Soda and pour them in shallow bowls or paper sacks and set them out in the car. Empty them out weekly and add more. Do this for a few weeks and see if it helps.
You may also need to remove the carpet from the car and take it to a coin op car wash, hit it with the high pressure soap and then the high pressure rinse. It should be safe after this to use a wet/dry vac or an extractor to remove excess water. Leave it out in the sun if possible to dry completely before putting it back in the car. This method is extreme and is time consuming but perhaps the best way to get rid of the smell.
Upholstrey, after a few weeks of the Baking Soda, can be dri-cleaned with something like Blue Corals Foaming Carpet Cleaner. Buy the one with the odor nuetralizers.
Hope that helps,
Anthony