Disclaimer: I lack familiarity in this matter, and this is only a conjecture about my potential course of action. However, it may prove beneficial.
Upon activation of the ignition, whether in accessory or start mode, the ECU engages the ignition buses. I suppose it does this via engaging a relay. Relays are located in the fuse boxes of the engine compartment. I estimate there are around four IG buses. That will be my primary objective. I cannot specify which one it may be, but if it is obstructed, it will undoubtedly result in your symptom. You could troubleshoot the issue by exchanging relays, one at a time.
A 15A radio fuse is located in the huge engine compartment fuse box. Merely pulling it will not rectify your issue, since the source of the problem is inside the feed to that fuse. However, you may see that voltage when actuating a relay.
If it is not a relay, the only alternative is the ECU. I am uncertain how to resolve the issue apart from a costly replacement.
Best of luck; nevertheless, do inform us of your findings.
