Dear T.:
I would try pipe cleaners drenched in rubbing alcohol. Try to wipe as deeply in all the vents as you can. Then I would shampoo the inside of the whole car (carpet, seats, etc.). When you are done, spray Febreeze on everything and into the air con vents. Let the air con run for a while with open windows to get the staleness out, then turn it off and spray air fresheners into them. You won't get all of it out, but it should be a lot better. Of course, you can ask a car repair shop whether they can somehow clean those ducts - they may be better equipped.
Stupid question: Have you checked whether the car has an ashtray? It might be full - clean it or toss it. Once the source of the smell is removed, it cannot stay forever.
I use a small air spray for the car and room sprays from the dollar store. It makes the car smell better for a while, and eventually it should fade away.
I tried the Ionizer from Sharper Image for the car. That is pretty useless. However, the home edition works well (you may have one of those already), perhaps you can run the air con with the Ionizer in the car via extension cord in the garage. It should extracte particles from the air, and again what is gone is gone.
Good luck,
W.
Good luck,
W.