Thanks very much for the guide and the help available on here. If anyone is starting this thread from the end then some very basic advice would be not to over think it, you don't actually need to know why you're doing everything. Reading through the thread can be confusing at first. The most important thing in hindsight is not to cut any wires unless they're at a plug. If joining anywhere else in the loom just splice into the wire as it could be connected to something else further up.
Does anyone know if the fan is controlled by the ecu in the SR20DE (S14)? I found some information saying that the relay was triggered by pin 9 on the ECU but the pinout diagram that seems right for my car has pin 9 as being the AC cutout relay. There is no mention of an engine fan. The car did have an electric fan as well as the viscous one.