Only thing I can think is to check and see how much slack is in the cover. You might have enough slack to start the cover, open a hole, attached arm rest and reinstall seat back before closing the cover. Some seat sovers are "open back", are these?
The hole will have to be big enough to accept a plate that is approximately 2 1/2" x 1 1/2". This plate has a notch which fits on a pin that protrudes just below the arm shaft. This arm is spring loaded and the plate provides the platform for the spring action.
'73 280SEL 4.5 (9/72)- RIP
Only 8,173 units built from 5/71 thru 11/72
'02 CLK320 Cabriolet - wifey's mid-life crisis
2012 VW Jetta Sportwagon TDI...at least its a diesel
Non illegitemae carborundum.