Over the last couple of weekends I built a new wooden frame for the rear seat base. The old base was a lot like the old front seat base: It had been somewhat crudely rebuilt by a previous owner, with about half the original wood left. I used the same techniques (and special jig) that I used last year to make a new front seat base. The new base is made of oak, and has all the same joints and dimensions as an original.
I also got some of the correct spring wire and metal clips for fixing up the rear seat springs. They are mostly OK, but some of the wires have corroded or worn spots, and could break at any time. After the springs are fixed up, I can upholster the seat base and repeat the process for the rear seat back. Fortunately, the original rear seat back wood is in excellent condition.