Notes:
1: North American cars had two fans connected in parallel other markets only have a single fan.
2: All 4-cylinder cars have an in-line fuse in the fan circuit. This fuse is fed from the white/brown (ignition relay but see below) circuit then feeds the thermostatic switch with a green wire. Be aware that this green wire is nothing to do with the main green circuit that is fused from the 2nd fuse up in the 4-way fuse block.
3: It seems that early cars with electric fans may have had a fan relay before they got an ignition relay and when they got the ignition relay the fan relay was deleted. The Parts Catalogue shows a 3-terminal relay the same as for the V8s but in order to use this the sensor switch would have to be wired differnetly probably he same as for the V8s.
4: Some owners have moved the white/brown wire for the fans from its usual position on the 4-way fuse block to a spare brown spade. This results in the fans continuing to run when the ignition has been turned off or indeed starting to run after you have left the car. Nothing earth-shattering in doing so - except that a fault could cause the fans to flatten the battery or in the worst case start a loom fire.

Notes:
1. In this circuit the fan wire is moved from the thermo switch contact to a relay contact. When the thermo switch closes it extends 12v from the green through to the relay which operates to a ground. The green on the thermo switch contact is extended to the other relay contact which when it closes powers the fans. Thus the same green feed is used to power both the relay and the fans but the thermo switch only carries the light current of the relay the relay carries the heavy current of the fans.
2. You should not need to add a relay to 4-cylinder cars with standard fans even two as the standard switch seems more than man enough for the job. However if replacement radiator switches fail quickly they may like replacement brake light switches be of poorer quality than the originals in which case a relay may be beneficial.
3. The relay contact number given are for current after-market relays. If using a standard Lucas relay use W1 for 85 W2 for 86 C1 for 30 and C2 for 87.

Notes:
1. Factory V8s have two fans wired in parallel.
2. The original relay is an unsual 3-terminal design in which the green supply from the fusebox is used to operate the relay in series with the sensor switch to ground as well as being extended through the contact to power the fans. This puts a heavy additional load on the green circuit - about 10 amps in my case - fusebox and ignition switch. The heated rear window also a high current item does the same thing. Not a good idea with 30 year-old electrics and a significant contributor to non-flashing indicators.

Notes:
1. Originally I though the V8 diagram was a misprint as mine has a four terminal relay with a connection from the brown circuit at the fusebox to the contact and hence the fans themselves meaning that only the load of the relay winding is on the green circuit. A useful modification but it should really be fused for safety either as shown with an in-line fuse in the wire to the brown circuit at the fusebox or an aftermarket fused relay.
2. Even with this brown feed instead of the green there is significant volt-drop in the fan grounds which share a relatively small-gauge wire with each other and the headlights right back to a grounding point by the fusebox. I provided an additional heavy gauge ground connection to a lug under one of the mounting bolts to the bonnet slam-panel for each motor. As my alternator has a spare large output spade and a spare input spade on the relay I provided a heavy gauge brown wire between them to increase current still further. These changes supplied an extra 25% or so voltage to the motors which gives a very noticeable increase in fan speed and hence cooling. In this case you either need an in-line fuse in each brown feed join the two browns together and then via a single in-line fuse to the relay or a single in-line fuse from the relay contact to the black/green fan wire.
3. Thanks to Graham Cornford for pointing out the error in the relay terminal naming. 4. If using a modern relay the terminal numbering would be W1 = 85 or 86, W2 = 86 or 85, C1 = 87, C2 = 30.