Ecu “nearly” always switches the earth.
so supply the relay with a cable of at least 2.5mm2 a link from this pin to energise the relay. The ecu will switch it on and supply all the switched lives for the engine.
if you used a 25a switch you could bin the relay of.
I just use a master relay as my Sierra ignition switch is 30 years old and not designed to carry the current.
If you have the fan switch in the rad or top hose you can bin that off.
If your not fussed about immobilising the starter motor you can bin that one off as well.