This is a cut and paste as well from another user:
I simply cut back the wire insulation cover on the cab-side fan wire connector at the top of the fan shroud about a half inch and used a yellow Posi-Tap connector to “T” into the dark blue wire. (PIC 1) I then ran a length of 16ga. wire long enough to reach the customer provided pass through wires (r,w,b and black) taped over on the wiring loom above the master cylinder. I connected the wire to the blue pass through wire with a take-apart quick connector so that it is very simple to pull apart the wire to do work on the motor or air delivery components.(PIC 2) Inside the cab I located the blue pass through wire (just above the emergency brake foot pedal) Extending that wire about 2', I added a simple 2 post toggle switch (for a ½” hole). To the other post, I added 2' of wire and terminated it on a 1/4” ring/eye electrical fitting.
At this point it made sense to check the integrity of the wiring assembly with an electrical meter. With the truck running, the meter from the ring fitting, to a ground point anywhere on the metal dash support bar exposed by removing the dash fuse panel cover (seen in PIC 3), it should read somewhere over 12 volts (whatever the alternator is putting out) with the toggle in the on position. And, it should read zero volts with the toggle off. If that works, then it is only necessary to connect the ring-eye to a good solid ground to complete the mod..
I found the small black screw at the lower left corner of the open fuse panel compartment to provide a very good ground spot. (PIC 3, black wire) The screw mates with a copper bracket that sandwiches the steel dash support bar. I located the toggle itself just to the left of the steering column, about an inch or so, in the part of the dash that sticks out above the fuse panel cover.(visible in PIC 3) It's out of the way, yet still easily accessible. The red wire (seen in PIC 3) is the extension of the blue pass through to the toggle. (red is simply the only wire I had on hand).
That's it, very simple and inexpensive. At least one way to do it.
But, I have some comments or thoughts and notes on this mod, now that I have made it.
1). I suspect that the manual override should probably be used sparingly. Generally, the PCM controls the fan speed based on parameters for the balanced needs for cooling and the horsepower requirements of the fan. [I read in a tech thread that at full fan engagement it can take up to 27 hp to drive the fan whereas it may be as low as 1 hp or less at the lowest engagement level of the viscous clutch.] Whether or not that figure is right, I think everyone can certainly feel some sizable drop off in acceleration power when their fan is fully engaged. The majority of owners who don't have detailed temperature monitors and gauges probably get along just fine with only the PCM fan control. Those of us that need to monitor everything tend to think we can do better than the PCM! I think the only real advantage for manual control is that we can potentially foresee upcoming events that the PCM can not possibly know. (like that there is a steep grade ahead or that we are terminally stuck in slow traffic at high outside ambient heat and aren't going to need that much hp.)
2). NOTE: Once you have made this mod you will not immediately notice fan engagement at idle in your driveway. But on the highway under throttle you should notice a substantial sound differential, especially on acceleration.
3). The mod will likely cast a P480 soft code, mine does.
4). In the toggle OFF position, the PCM control of the fan should not be effected by this mod. Therefore, the flexibility to control the fan engagement “full-on” manually is worth the effort, I think.
5). I put a length of protective flex tubing over my wire in the engine bay to protect it against heat and possible short. But that is probably superfluous since if the wire did ever ground out all it would do is engage the fan, which should be fairly obvious to the driver and easy to fix on the road by just disconnecting the blue wire tap at the“T”.
6). I think that there is an advantage to a manual switch, if for no other reason, to be able to reassure yourself periodically that the fan clutch is still operating correctly.
7). All of the talk in this thread of trying to develop an adjustable manual fan control is in my judgment, overkill, as the PCM can more likely do a better job of controlling variable fan functions in most instances. Certainly better than I could.