First get the carpets steamed...that should help a lot.
Next, pull the seatbelts all the way out and leave them out all night for a few weeks with the windows open (hopefully you have a garage).
Then, get some Mother's leather cleaner, scrub it in with a toothbrush and wipe off with a clean rag...do small sections at a time.
To get the leather smell back, get a hold of Tannery leather conditioner...it is in a black aerosol can. I get it at Wal Mart up here in Canada. Spray it on the seats after they have been cleaned. Wipe off with a clean rag.
That should do ya...!!!