Voltage drop question

I've got a voltage drop problem and am looking for ideas.

I'm using a 4-pin ECU and a 2-connector ballast.

With the key in the RUN position, I measure about 5 volts at the coil or the ballast resistor, with it in START I get about 8 volts.

When I unplug the ECU, according to my notes I get 12 volts in either START or RUN, which doesn't entirely make sense why it wouldn't be less in RUN but maybe I measured from the wrong terminal. Either way, there is a huge voltage drop with the ECU connected, and it goes away if I disconnect it.

I double-checked all the ECU connections. It takes 12v off the blue wire at the ballast resistor, as it should. The ECU harness is new.

I replaced the ECU and ensured it is grounded. No change, still a big drop with a different ECU plugged in.

The car has been in the process of getting a new engine for a while now, but AFAIK it was doing this before I took it apart with the exception that I didn't check it with the ECU unplugged. It did start and run though.

I'm hoping to get the voltage up since it's time for first fire-up and I want to be sure everything is as right as it can be before trying that.

Any suggestions on what could be wrong or things I should check are appreciated.