Dothe Dew's diagram is incorrect.
The aux fan on the new models is controlled by the ECU, and the ECU (or PCM) does power the fan by activating a relay. That much is correct. According to the 2000 FSM, though, the relay (which is in the power distribution center) does not have the terminals numbered like a standard relay. The FSM uses numbers D10, D11, D13 and D14. These would correspond in the diagram above to 85, 86, 87 and 30. 87A (or the equivalent, which is not numbered in the FSM) is not connected to anything.
PCM to D13 (85 or 86) is an 18 gage wire, dark blue w/purple (pink?) trace. The other side of that circuit is D11 (86 or 85) and goes to ground.
Battery in for the power side of the relay goes to D10, which corresponds to 30 on a standard relay. Power out to the fan is D14, which would be 87 on a standard relay.
Folks, this setup is NOT negative switched. There's a 12 gauge light blue wire from the relay to the fan, and from the fan there's a 12-gage black wire directly to ground.
This means that if you wire in a single pole toggle switch to override the PCM and turn on the relay ... you will be tapping in between the relay and the PCM, and that means you would be back-feeding 12 volts into the PCM. Whether or not this would have any effect on the PCM I don't have a clue. Don't want to find out the hard way.
Simple solution: As noted, a SPDT toggle switch (single pole, double throw).
Cut the wire from the PCM to the relay. Connect the middle (common) terminal on the SPDT switch to the wire leading to the relay from the cut. Connect one of the switched terminals on the switch to the wire leading back to the PCM. Connect the other wire to a fused 12-volt source. Switch in position A gives you stock operation, switch in position B isolates the fan relay from the PCM and uses direct 12-volts to activate the relay. No back-feed.