W126s as well as W140s could be factory ordered with two types of rear sunshades. One the regular shade that travels up and down (it comes in both, electric and manual operation). I'm not sure about the W126, but rear seat console on W140s must also be replaced to install the electric sunshade.
The other is the type is the electric curtains, which open to the sides (theater like).
The electric sunshade can be directly ordered from M.B. using the following part numbers:
Installation kit for the electric rear window sunshade:
B6 668 8410*
*until production date 09/85 part # A 126 683 09 00 must also be ordered.
Shade itself to be attached to the kit:
Black: B6 668 8411
Blue: B6 668 8432
Light tan: B6 668 8414
Mushroom: B6 668 8438
Medium red: B6 668 8416
Pine green: B6 668 8419
The SECs use different part numbers.
CL 500, 1998
S 500 L, 1998
E 320 T, 1995