New harness problem

OK, the yellow comes FROM the ignition, and when you twist the key to "start" the ignition switch provides power down the yellow from the switch, through the bulkhead, to the start relay.

This is a change from what you had listed in your diagram. There you claimed you DID have power from the yellow, and power at the starter relay on the brown. This would indicate that the starter relay was getting power, was "pulling in" and was closing the contacts to the brown.

You need to find the problem BEFORE you jump on the start relay.