If your rear seat upholstery is still intact, you could try removing it from the springs and making up new padding yourself. here's what I've done in the past. First a layer of thin carpeting over the springs to prevent them from cutting through the new padding. The thin indoor-outdoor stuff, as used on patios and balconies, works well and can be found at Home Depot. On top of the carpet, add as many layers as needed of polyester fiber pillow/quilting padding, availible at fabric shops. For a firmer padding, instead of the polyester you can also use layers of carpet padding, availible as foam rubber or recycled fiber from places like Home Depot.
If possible, try to arrange the padding to restore any original seat contours. You may need to use thread, wire-ties, staples, string etc to secure the new materials to the springs. 3-M aerosol weatherstrip adhesive can be used to secure the different padding layers. Then reinstall the upholstery. It may not make the rear seat perfect but it should be useable.

Happy Motoring, Mark

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DrDKW