Viewing seatbelt reel
To be able to actually see the reel, all you need to do is to remove the rear seat. To do this, first remove the lower cushion by pushing in the two orange levers at the bottom front of the cushion, then lift the front of the cushion and remove the cushion from the car. Next, remove the 3 screws that hold the seatback in place. One is dead-center, the other 2 are out at the sides. With those screws removed, lift straight up on the cushion and unhook it from the chassis...it lifts out pretty easily. That done, just pull back the wool liner behind the seatback cushion and there's your reel. If you need to replace it at that point, it'll be a piece of cake. Good luck.
PS There are no ETRs on the rear belts of a W124
"If everything seems under control, you're simply not going fast enough" --Mario Andretti
1995 E500 street car
1986 Porsche 944 Turbo S track car