My girlfriend thought that I had developed some kind of tick, as I could not sit still in the 'Bird. Turns out that I was constantly squirming around, in an attempt to get comfortable, as the seat foam has lost much bounce. My *** was almost on the carpet.
At Target Department Store I purchased a pack of cotton furniture padding, and some mono-filament fishing line.

GM used wonderful seat material. Wears like iron. I sewed some seams with the fish line. Then I installed the padding to firm up the bottom and back cushions. Also vacuumed under the seat, for the 1st time.

Turned out nice. I lucked out. Cost next to nothing.
I know, "Get a digital camera."
Now I need to get a sheepskin cover, to protect the thing, before that old material does wear out.

Seth