Like many E39s in this day and age, my quarter window sunshades did not retract and flopped around. This is due to the plastic piece that holds the mechanism in place breaking. My design takes the place of the old broken plastic.
Step 1: remove sunshade from quarter window
Step 2: dremel out remains of broken plastic around the rectangular rod
Step 3: tension the sun shade by rotating the rectangular rod to pull it in
Step 4: insert the printed piece around the rectangular rod and into the sunshade assemble and glue it in
Step 5: Flex on your other E39 friends with bufu broken sunshades either flopping around or held in place with paperclips

Could have been done a bit better, maybe to fill in the entire area but this worked great for me. Feel free to remix/ improve/ whatever.