I have a 1968 firebird convertible with the fold down rear seat too. But my seat doesnt fit the car so it must not be original to the car. I want the original seat. I heard the convertible didn't come with a fold down rear seat. Do you still have the old springs for yours?
As far as I know the convertible was available with the fold down rear seat. The only differences between the fold down seats is that the 1967 version has a "slam latch" while the 1968 and 69 use a release lever on the driver's side. Is your seat too wide for the car that's why it doesn't fit?
