If It wont fire until the key is released the blue/yellow circuit to the ECU is dead during crank. Where? It depends if how it was wired when installed. Where was the wire attached to the ignition on circuit? Be aware that Mopar has 2 ignition circuits. Ignition 1, is hot in run but not crank (I believe red on your truck). Ignition 2 is hot in crank, but not run (I believe pink on your truck). The best way to find voltage drop? Connect a voltmeter hot to hot (positive side drop) or ground to ground (negative side drop). In your case battery positive post to the Blue/yellow wire. Crank the engine. By all rights the voltmeter should read zero. It won't. What ever reading you see is how much voltage is lost between the circuit testing point and the battery positive post. Don't be surprised to see readings of .5-1.0 volts. More than that is an issue. Start at the battery terminal and work your way down the the circuit. Do this test while the circuit is loaded. Such as while cranking, while running or with what ever component you want to test is switched on.
Doug