Entire ABS system has a special relay on the CEB:

ABS-relay (relay XVII up to MY 1990 or XV on CEB starting MY 1990)

Relay Operation

ABS-relay has overvoltage protection based on a Zener diode. When reaching an overvoltage of 22 V (e. g. defective alternator), this diode will be conductive and prevents further increasing in voltage. This protects control unit against damage from excessive voltage.  When the Zener conducts the #28 fuse (#16 prior to MY 1990) will blow which prevents worse.

Power on Bus15, switched 12v [power from turning ignition key to on] makes the ABS relay activate and transfer the main 12v power from the relay panel to the ABS, ABS ecu and RDK ecu.