How you wire up these circuits depends on what type starter is in the car.

On the Fords - the old large starters are a one wire starter. The large (4 gauge) cable running to the starter only gets power from the fender mounted switch when you hit the 'start' position on the ignition switch. The fender moounted device is a just a big relay. Hot from the battery on one side; big cable to the starter on the other. When you hit 'start' by twisting the key, you supply 12V+ to the small terminal on the fender switch (powering up the coil in the relay) and 12V+ is sent to the starter turning it over.

On the smaller mini-starters - it works entirely differently. They are internally switched and have a 2 wire hook up. So the big cable going to them is hot all the time. The small wire going to them should be hot only when you turn to the 'start' position. This triggers the starter's internal relay and allows that switches power from the big cable to turn the starter and engage the flywheel gear. So with the mini-starter you can use a traditional Bosch relay as described by swede, or you can use the typical fender mounted relay. To use the fender relay with the ministarter, put the big cable on the same terminal as the hot cable from the battery (big cable hot all the time), and put the small wire from the starter on the other terminal. The signal from the start/ignition switch still hooks onto the small terminal on the switch.

Clear as mud?