What Circuits For Relay?

If you add a lot of high current draw devices [ elec fan, air con etc ] without using relays, then you would have to upgrade wire thickness to carry the increased current, otherwise you will get large voltage drops which may affect the operation of the devices. It is not practical, in fact dangerous/risky [ like a fire ] trying to pull large amounts of current through the terminals of bulkhead head connectors & ign switches that were NEVER designed to carry those large currents.
Use relays. The typical Bosch 30 amp relay has a coil resistance of about 70 ohms. So it draws less than 0.2 amp & will not overload existing circuits or wiring. Use the relay contacts to do 'the heavy lifting'.