I just bought a 1977 Camaro and it needs a little TLC. There is a piece of plastic in the back but I dont know what its called. So Picture yourself sitting in the left rear seat, And look right to the left of you, There is a peice of plastic. It dryrotted and I want to replace it, but Like I said IDK wat its called.

 

I think you're looking for the interior side panel. They attach between the door and the wheelwell inside, and have a armrest built into them.
Is this it in the far side of this picture?

 

Yup thats it, Thanks.