All right "massive cable" does not tell me much. Your idea of "massive" and mine might be different
Detail the path from battery to starter. Is there a disconnect? Second solenoid?
What is the ground path EXACTLY. Might be as simple as a poor ground between engine/ body/ battery.
Before you do anything, and after the car as sat awhile (to bleed off surface charge) measure battery terminal voltage and post it here. Maybe? the battery is down
Clip your meter to the starter big main terminal on the solenoid, and to a good engine ground. Crank the engine and read the meter. No disconnect the coil wire and ground it and check it again..........starter may be "kicking" on the starter. You need "accepted" minimum of 10.5V and anything below 10V is just not gonna fly
