Oh, that. Wikipedia at one time had no end of trouble from the "hemp car" enthusiasts, until finally documents were dredged up from the Henry Ford Museum.
In the early days of plastics, there were lots of attempts to find some filler that could be dumped into the mix to save money. Sawdust works, as do other plant fibers. Cotton is still used to improve resilience - pure phenolic is brittle.[1] Fillers with plant materials absorb water, though, so they don't last well in auto body panels. Glass fiber and carbon fiber are still used. Asbestos filler has been phased out.
I feel like this article ends halfway through - with the pictures of the car and...that's it? No explanation if it's real, if the panels were indeed made from soy, if the factory actually produced anything...it started with a mystery and then nothing?
In the early days of plastics, there were lots of attempts to find some filler that could be dumped into the mix to save money. Sawdust works, as do other plant fibers. Cotton is still used to improve resilience - pure phenolic is brittle.[1] Fillers with plant materials absorb water, though, so they don't last well in auto body panels. Glass fiber and carbon fiber are still used. Asbestos filler has been phased out.
[1] http://www.reblingplastics.com/phenolic.htm