Musty smells
I have another solution that I have had success with over the years.
If you can contain the seats in some way, i.e. put them in a plastic bag or cover them with something that will not allow air to circulate.
Prior to doing the above, stuff any open area's of the seats with waded up newspaper or advertising inserts. Also cover the seats with the same type of paper prior to sealing the covering.
The ink used in the printing process will absorb the odor.
This process is excellent for coolers, refrigerator's or freezer's that develop an odor.
I once had a very large freezer lose power while I was away for a month. The freezer was full of meat and developed a terrible smell while I was away.
Upon my return, we emptied the freezer and stuffed it full of newspaper. We then closed the door and let it sit for a week. When we opened the door and discarded the paper the freezer was free of any odor.
I have also used this method on cars that are stored for an extended period with the doors and windows closed. The only problem with that is it takes a ton of waded up paper to cover the floor of a car.