Sorry I see it now. Reread 737 and Dave recommendations. I’m not sure where your junction point is looking at you picture . If it were mine I would hook the battery + cable to one of the large terminals of the starter solenoid. While you’re doing that slip your 40amp fuse + supply to the mUnit on this same terminal to keep your runs short. If it keeps shutting off after this you’ll have to disconnect each of the power wires you have at the m units positive terminal, your using as a common junction, and checking each for ground potential..
Keep working on her, she give it up. She’s just making you work for it.