I was thinking about this today on a way to test the grounding. If I recall, the ECU will ground the coil "-" terminal till it sees a pules from the distrubitor pickup?
I think if you unplug the dist pickup, the ECU should keep the coil "-" grounded while cranking the engine.
So try this test:
#1 - unplug dist pickup wire.
#2 - Connect a good voltmeter (positive/red meter wire) to the ignition coil "-". Connect the meter negative/black wire directly to the battery "-" terminal.
#3 - jumper a wire from the battery "+" terminal to the ignition coil "+" terminal.

Test: The voltmeter should read nearly zero volts. The only voltage drop should be the ECU transistor junction voltage (maybe less than 1 volt?)
Now crank the starter to put a high current load on the wiring. Because the dist is unplugged, the ECU should remain grounding the ignition coil "-" terminal. Ideally, the voltmeter reading should be the same as before. If the reading goes up, it is because of resistance in the ground path(s).