I don't have one for sale, but I think you could make one very easily. I don't know if mine is stock or not (never seen one) but it appears to be made out of 3/4" plywood for the base and have the cloth/padding stapled to it. I just got home from work and am beat but will try to get some pictures and dimensions posted for you if you like...Only part that might be kind of a pain IMO would be piano style hinge for the fold down portion...my seat base is removeable to allow the seat back to fold flat..
Hey Travis: I kind of forgot to take some pictures/measurements for you but here there are incase you wanna try making your own back seat.
Seat bottom is 18"x36" and using 1/2" plywood, not sure how much foam, seems like 2" thick. Kids (10 year olds) seem to think it is ok for a couple of hours The seat back measured 14x38" (1" overlap on each side to allow it rest on the lip) and uses a 3/4" square tubing frame. The fold over part is 15x38" also made of 3/4" sq tubing. It is attached to each other with a piano hinge where it folds and also to the bed of the truck. Not sure how much steel is there. As you can see by the one picture mine needs to be redone at some point.... I am debating about making a split frame to allow for one side to fold down for 3 passengers and more space for camping crap..
Please ignore my plexiglass speaker hole covers--another thing on the list to fix... Hope this helps you out in your quest for some back seats...
my truck has an original back seat. if your a die hard for the pattern and such can send some photos. they were originally foam covered 1/8 laminated board( like panaling) that was velcrode to the fiberglass floor so it could be moved forward so the back lay flat.
'83 trekker v8 conversion '85 pick-up v8 conversion '89 forerunner last of the removable top too many put in the grave to count!! will resurect as many as possible til i die