There are two ways to do this.
First way is use one main relay rated for total amperage usage. This will be turned on by the ignitions switch moving all current usage to the relay. The relay will supply power to the power strip. Each application will need its own fused circuit and proper wire size coming off of the power strip. The power strip will need full amperage usage fuse and wire diameter.
The second way is to turn on the power strip right from the ignition switch. Then add relays from the power strip to each application using the proper wire diameter and fused power source. This system requires addition wire from the main battery source to power each relay, but you can tailor each circuit according to its needs. A single point power feed right from battery will be needed to power each relay.
Both system requires minimal current from the ignition switch.
For cooling fans, the relays are usually mounted out near the fan with small wires running them from the switch panel. The power feed is right off the alternator or battery with heavy wire to the relay then a short wire to the fan. Race cars do the same with electric fuel pumps, relays mounted at the pumps so the motor draws power through the shortest, heaviest wires possible. The long run, small trigger wires are less likely to burn much if they get shorted to ground with 3 or 5 amp fuses.
