I had to replace a broken medallion on the drivers seat on my R/T recently. The bendable metal attaching strips broke off on the back of the medallion. I think this is a common design fault on our now 55 year old cars. This is a hard fix with a good spare medallion, if you are doing it by yourself because the tabs need to go through the seat vinyl, foam, burlap, springs, and metal retainer before being bent over to attach the medallion.
I devised a way to do this by myself, using some scrap wood around the shop. I used a base piece of 2 X 4 longer than the diameter of the steering wheel, to which I screwed a piece of 2 X 2, just shorter than the medallion in the centre of the base piece. I took the back off the bucket seat and retrieved the metal retainer. I wrapped the ends of the 2 X 4 base so it would not mark my steering wheel as I laid it across the wheel. I had fed the replacement medallion tabs through the punched holes in the vinyl seat cover. By moving the seat all the way ahead, I was able to climb in the rear seat area to place pressure on the front seat against the carefully placed wooden fixture so that the 2 X 2 piece forced the medallion and tabs far enough through the seat that I could bend them over again through the metal retainer. The metal seat back was replaced and the repair was completed.