You should be able to tell if the seat spring is shot. If when you sit down and the seat gives way the spring is shot. If you're not sure, compare it to the drivers side. As far as what's under the cover, there is a horse hair pad that gets brittle and disintegrates over time.
You may be able to get one here, but if not, try
www.************************. They sell the springs and pad. Together hey will cost around $150. It will make a huge difference.