Having wired a number of cars and done a lot of wiring, I will say that no harness is really plug and play except a stock harness in a stock application and even then you probably have to "adjust" some things. In the cars I build now, I buy a generic harness and then reconfigure it to suit the vehicle, that said you have to adhere to some wiring principles to get it to work well and last a long time. The principles are varied but simply put, all of the terminals must be well connected (I crimp and solder and heat shrink all my connections), all auxiliary components generally run off relays especially high draw things such as lights, fans and fuel pumps. I build connection points to connect all of my accessories that are not directly addressed through the harness, everything is fused, the gage of wire is important to keep things running/working over time. Grounds are imperative, I install 4-5 grounds throughout the vehicle (engine to frame, battery to frame or engine, dash to cowl, fuel tanks to frame, firewall to engine, etc.). I protect all of the wiring to ensure it isn't near any heat source or rub points and use grommets whenever passing through sheet metal.
My point here is that wiring while simple in concept can be complex in application and there are many "mechanics" who are not at all adept at doing it. Many feel that a 12V source is a 12V source and therefore can be tapped into without issue. They do not factor in resistance, draw, etc.
With your fans, as noted there are 2 12V requirements for a relay; one is what I call a "feed wire" and is connect directly to a battery source, usually has a heavy gage wire such as a 12 or 10 depending on draw from the accessory/device. The other is the "trigger" wire and is connected to a switched source meaning that it does not have power unless the ignition is on. Normally your on/off switch is connected on this circuit.
