Used a small hand held one on mine but only on a few areas and wiped off the residue with a microfibre.
The Karcher hand held jobbies were £29.99 in homebase when I got mine a few weeks ago. Great to use on headlining and carpets too. Probably seats as well if you haven't got leather.
Through being a smoker in my car for 3 of the 6 years of ownership, I got so much black out of the head lining from the steam cleaner and microfibre, even though this has previously been cleaned with interior cleaner.