Please answer the questions that we already put to you. What are your observations on the dash, with the RID?
Have you cleaned the battery terminals? Provided an alternate ground path from battery to the chassis? What is the battery voltage?
With this "rapid fire" is the fuel pump cycling on and off?
The Motronic has to see a pulsing input from the HES to fire the injectors as far as I am aware. Normal operation of the fuel pump is one short burst with each on/off ignition cycle. If you switch on but do not start the engine there is no further activity from the pump.
I do not think that the initial pump priming is accompanied by an injector pulse.
If the ignition power is intermittent because of a loose ground, bad ignition or kill switch then the fuel pump prime might repeat.
You have several suggestions to follow. Please do those first and report your observations before we move ahead.
If the crank is stationary I do not see how the injectors can receive a pulse from the ecu. Is anyone aware of Motronic firing even one pulse to injectors at power up if the crank has not moved?
Restated: the pump will power up briefly (a second or two) each time the ignition is switched on, even with crank stationary. If you repeatedly cycle the kill switch on-off-on the pump prime repeats.
Very few Motronic ecu fail catastrophically. I have never heard of one failing in this manner which allows repeated injector firing. I likely have hearing loss because I've never heard any noise from my injectors, then again either the starter is cranking or the engine is running when injectors ore powering.