the starter solenoid acts as a giant 300 amp relay to energise the starter - the coil in the solenoid (aka terminal 50) sucks about 40 amp on contact and stabilises at 10 amps - 40 amps on the starter switch is nasty
a bosch 70 amp relay coil only requires 0.5 amp current to energise and pass the solid 40 amp load to terminal 50 from the battery within 2 ft of wire.
Thats how Ive seen all 12V Toyota and Nissan diesels wired up from the factory
