I'm stumped on this E-250 fails to start.

[DAV!D]

Ok, well I double-checked the ignition switch by manually activating it and it still won't engage the starter so it's not the key mechanism and the fuel pump does turn on with the key.

The only thing I have left is it has to be a wiring issue somewhere because I've checked or replaced everything else.

 

[Bikersmurf]

As mentioned previously, did you replace the relay that’s on the fender behind the passenger headlight?

There’s also the starter solenoid on the starter… I’ve had the one on the inner fender fail.

 

[G35Vortec454]

OK, rather than looking for a chafed or broken wire, I suggest connecting a wire to that 10A fuse terminal with the other end at the starter relay. If that doesn't work then your theory about power at the key start pin is incorrect. If it works then keep the jumper wire and cutoff the "broken" wire.