On my '87 420 (also a 126), the sunroof is at fuse nr. 19, 16 amp red.
The sliding roof can also be operated manually as per Owners Manual page 85: Fold back access cover in left side panel of trunk with a screwdriver. Insert socket wrench from tool kit through opening in panel and place on the hex-drive of the electric motor. To open, turn counterclockwise ...
Hope this helps.