Voltage drop at ballast resistor

Mattax said
'72 should be a dual ballast resistor. What is on your car?
The 5 ohm resistor has a wire going to the ECU - it will show a big voltage drop.
The 0.5 ohm resistor should have the wire going to the coil - it should not show nearly as much voltage drop.
One possibility is these are connected to the wrong resistors.
Another is that something in the coil or the ECU circuit is a drawing a bunch of current.

The drop to 10 volts from 12.5 even with the resistor's feed side (probably blue?, or blue with trace?) is something else. Power is going somewhere and its not the ECU or coil. See if disconnecting the alternator's field ground (green wire at the alternator) changes that reading. If not, try the field positive (probably blue?).

OK, I performed the steps you suggested below:

The drop to 10 volts from 12.5 even with the resistor's feed side (probably blue?, or blue with trace?) is something else. Power is going somewhere and its not the ECU or coil. See if disconnecting the alternator's field ground (green wire at the alternator) changes that reading. If not, try the field positive (probably blue?).

With the green wire or blue wire disconnected I read 12.45 volts at the blue wire (disconnected) to the ballast resistor. when I connect the blue wire to the ballast with either wire at the alternator disconnected I get 10.45 volts at the resistor blue wire. Where do I go from here?