No a fan OVERRIDE switch can be a good idea but it's an automated system for a reason. You are on the right track take a feed direct from battery via 30A fuse to a SPST relay, you will need another live feed for relay as well then ground relay into one side of fan switch and run two wires to live of fans themselves and connect to switched live side of relay. Make sure the right you use is the right gauge otherwise the power the fans draw will melt piddy wire.

It's actually surprising if fans do work on original wiring if you were to look in your fusebox I bet there is a melted 30A fuse/holder there. Also the fuel pump wiring degrades over time so it's worth rewiring this as well.

Martin