The Volvo relays are a great option, but everyone always forgets the Contour wiring system option. Why not run Contour fans the way the factory did (almost). The factory triggered high and low speed via the ECU, in our cars we need an external trigger, either the BMW switch Mustang5L5 mentioned or the Audi/VW switch I used or any of the other sensor/trigger systems out there. The beauty of using the Contour wiring and relays is you don't have to worry if the system will work - it did from the factory.
How is the Contour wired ...
This is how the Contour is wired with the exception of the fan switch here at the bottom and the starter solenoid. This is where you connect your sensing/trigger (whatever it is).
This past weekend I made a short video while at the wreckers showing what to take if you are picking one at your local yard.
If you are looking for more, you can grab part of (or all of) the fuse/relay box as well which will get you the Mega fuse and 70A relays.
Here you want the left half of the panel above. The yellow relay is used by Contour for A/C clutch, the two darker relays below it are the 70A units. If you can, remove the panel carefully and you can have the connection to the fans completely with OEM wiring and connections (it can be done, I have done it twice with my Mustang and F150 I converted to e-fans with a Contour control system).
If taking Contour fans and relays/fuse panel, don't snip at all. Take your time you can salvage the whole system from battery connection to fuse panel and fuse panel/relays to fans and ground connector. In the video, someone hacked to get it out and then just left it on the front seat. I have 3 other OEM Contour dual fans I am hoarding for future need with the wiring shown.
Factory wires undersized - NOT !!! Most people talk about 10 ga wire to power the Contour fans. Ford used 8 ga from relays to fans and 4 ga from battery to fuse panel. Beefier than any system I have seen proposed to install Contour fans in anything.