All 126 chassis central locking systems use the single line system. This system has a pump that reverses giving either vacuum or pressure to lock elements.
All US cars had the pump in the trunk in the spare tire. The early Euro cars had it somewhere else, can't remember where. It was usually only the 1980 models but a few late 1980 models got titled 81 when they were brought in.
Find the pump, get a wiring diagram and see if the pump is getting its simple signal.
Bosch Master, ASE Master, L1
33 years MB technician