Being the injectors fire, the CPS is doing it's job.

What is the history of this car?

These coils are waste spark so one end must be connected to a spark plug or ground for the other to fire.

Use a incandescent test light when looking for 12v , when using a digital meter a bad connection will show 12v but if any sort of load is applied the voltage goes away.

In another thread, someone lost the ability to talk to the ECM, it turned out that a ground was dirty. ( MB puts a bunch of grounds on the same terminal compounding the problem.)

Hook a test light across the injector plug to see if it still flashes.

If you swap ECM boxes, you need to get the DAS X module behind the speedo and a key that matches both boxes otherwise you will get a no fuel no start condition.