Never mind, I figured out how to get the back panel off. It's hot glued on plus the rubber edge of the leather seat skin also holds it on. I found that the retractor is not accessible from the back, the leather skin has to come off. What a pain. I am not the idiot who cut the belt.

Here is how I accessed the seat belt retractor. I unclipped the seat skin from tabs on the frame around the armrest opening and was able to see there is a steel cover bolted on over the seatbelt retractor. Once that came off the retractor was visible. I will have to source a donor seat back to steal the retractor from.