Buring wire on start up

Hi all, I have just in the last couple of days swap over a ej25 from one T3 to another. I pretty much copied it wire to wire, but tonight I just burnt out a black wire earthed next to the ECU. It only starts to burn when you kick the car over, but not when the reds are on. Any Ideas????

All good! I just remembered that I forgot to attach the earth strap on the gearbox properly. I guess it was trying to use that earth to activate the starter, hense the smoke...😂