If your test light is continuously lit while cranking on both sides of the coil then the ignition control unit is not switching. The + side should stay hot but the - side should be ground to connect the coil. The field is built during the brief time the coil is energized. During the energized time the light will be off on the b=negative side.
The control unit reads the speed signal from the inductive coil within the ignition distributor. If you have another car handy it would ease the diagnostic by having a good control unit to plug in.
To diagnose properly the speed signal should be monitored with a scope to verify. If the controller is powered and the inputs are there then it would look like a bad controller. Fairly common on that era controller.
Bosch Master, ASE Master, L1
33 years MB technician