Is this the sign of a bad ballast resistor ?

(1970 Challenger 440 Six Pack)
Orange box elec ign

Battery 12v
Ballast resistor (harness unplugged) 2.0 Ohms across the terminals.

Key on engine not running.
Ballast resistor harness plugged in.

Voltage at bottom of resistor (input / blue white & green white wire): 10.6v
Voltage at top of resistor (output / blue & gray wire): 2.9v

Voltage at coil: 2.9v !!!!!

===> If if unplug the ballast resistor harness and measure the blue white / green white wire (input) I get 12v but it instantly drops to 10.6 once I plug it in and the output voltage is 2.9v

Is this a bad ballast resistor ?

Thanks in advance for any help.

Sounds like your problem is upstream of the ballast. You should have battery voltage to the ballast when connected.

The ballast drops the voltage to something like 6 to 8 when the engine is running.

I'd be tracing the wire back to the ignition switch.

I've never seen one be anything but open or closed. Run or not. But maybe someone else has.

That’s what is strange.
The voltage to the ballast (blue/white - green/white) is 12v until I plug it in, then it drops.

I'd be looking for heat damage at the firewall and ignition switch connectors.

If that is the old single piece Mopar dual ballast resistor that sounds correct as far as once it is warmed up, both sides are wire wound resistors one has more winding than the other hence the difference in ohms and voltage drop through the resistors. Wire wound resistors will change ohm readings and voltage loss when warmed up
What problem are you having with the car?

Except his description indicates the low voltage on the input side of the resistor.

Not strange at all, it is perfectly normal. You are reading, with the ballast disconnected, what is called the open circuit voltage. As in with no load applied. Add a load, the ballast circuit, and your supply voltage drops again normal. In your case is should be darn near battery voltage at one end and something like 6-8v at the other. You have a source voltage issue, probably the bulkhead connector is dirty or the ignition switch.