SL/R129: 1990 300Sl 24V
I would check parking brake, doors closed, latch micro switch, seat back in locked position, window position. ( although our earlier models did not have the window sync issue of the 95's and newer )
I always warn about the fuse quality in older R 129's and support the concept of Al contact problems and for a few $ you can replace all the fuses with the Cu clad version of the ceramic bullet.
I would think wiring break after all the "latch" issues are sorted.
Good luck



