That is an excellent point...

Normally, I check voltage drop across the circuit rather than check voltages at various points.

In this case, a probe on the case and a probe on the engine with the meter on volts...then crank the engine and whatever voltage shows up on the meter is an indication of how good/bad the ground would be. Having the ignition module box behind the glove box makes it hard to check things without about three people available-or some creative rigging

By yourself, it is hard.

I still think this is a simple problem but it is being difficult to corner.