Rear backrest removal 63 Valiant convertible

Hi guys,
Im trying to get the rear backrest from a 1963 Valiant convertible out of the car. I know how to get the part of the rear seat that you sit on out, but want to take the back rest of the rear bench seat out. Tried pulling up, pushing down - but no go.
What am I missing??? Getting a bit tired of laying in the trunk!!!
Thanks