I'm assuming that the yellow/blue wire is the only small one at the starter, and it is somehow "live" all the time. Either that or the big red lug is somehow touching the y/b terminal.

I'd take the y-b wire off, connect the battery, then see if the y-b wire has voltage on it (seems like it must). If so, open the firewall plug (check that the y-b wires match up) and see if the y-b on the "car side" is hot. If it is, the problem is not in the new harness. If y=b is not hot, then there is a problem with the harness.

That's all I can come up with. It's a simple circuit: Ignition switch ==> Neutral Safety switch (if Auto) or jumper ==> Firewall connector ==> Starter terminal 50.

As for your other post, I'd say one thing at a time. Maybe even a separate post/thread on the fuel thing. And put car type and year on subject line like I did here (cuts down on irrelevant responses).

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Bruce Young
'93 940-NA (current) — 240s (one V8) — 140s — 122s — since '63.