Thanks for verification Vitaloni. As the 50 amp fuse blows instantly the possibilities unfortunately are many, a short on any of the spliced together ten red distribution cables mentioned above or an ignition switch failure even though it's off. Doubt the two relays mentioned above would be the cause as neither would be energised with ignition off and no permanent grounds on them to cause internal short.
Suggest a check for anything chaffing from 50 amp fuse to eleven way splice and on to each of the ten red cables fed from this point. If nothing found would turn attention to ignition switch as this is the only component fed directly by 50 amp fuse and permanently live to its input even when off, red cable to black connector. If you have a plentiful supply of 50 amp fuses pull off this connector and try with another fuse.
It's tempting to benefit from MTWV2 experience but his immobiliser was working with everything off and fuse only blew with ignition on if I understand correctly. For obvious reasons immobiliser requires a permanent 12 volt feed but this is fed by fuse 3 (15 amp) in main fuse box which would have blown saving 50 amp one given immobiliser short.
Sorry tracing of this fault is onerous but when you do find short it will be pretty obvious given its instantly taking out a 50 amp fuse.
BTW try as I might I cannot find the other end of the tenth red cable from main feed splice. It appears on wiring schematic for both early and late M3W coded R80. It could be a spare feed never adopted on loom. If this is the case the loose end should be securely insulated but who knows
