How can I take the back rest of a rear seat out of a 1970 Dodge Dart Swinger so I can put a seat cover on it?

If they are like C and B bodys, when you take the seat bottem out (push back and lift up at the same time) there should be a single bolt at each side going through a thick loop at the bottem of the seat back. Undo those two and lift up.

On my 71 duster - The bottom comes out first, you have to push down and in, then lift up. If it won't lift up, push down and in harder untill it clears the clip, one on each side of the hump. You can feel for the clip with your fingers underneath. Then for the back, you push straight up, then pull the top part back, and then make sure the frame loops clear the clips near the floor board in each corner.

The retaining "hook" on one side of my '71 Dart was bent to the side a bit (which wouldn't allow it to line up with and pass through / release from the "loop" part). Check for a similar situation on yours if it just doesn't want to "let go". I really had to put my body weight against mine (while pulling up, as stated by Pierre). Some cars are easier than others: just a matter of variation of factory fit, I guess. Good luck.

---Red