I've wired a switch to activate the fan whenever I want. It uses one of the two wires that connect to the fan.
Find some instructions for installing a hybrid battery charger harness. They will show you where those wires are. That's because those chargers need to run that fan while charging. The procedure will also walk you through pulling off the necessary cover panels.
My recollection is, one of the two wires is always hot, 12V (with ignition key in run position). The other is a ground that's essentially switched by the computer, using a couple relays. One relay is for high speed, one for low. So that wire acts as a ground only when the path is completed by one of the two relays.
To find the hot wire, probe the two fan wires, with the ignition switch in the run position. The one that doesn't have voltage is the ground. If you ground that wire, the fan will run at full speed. You can ground it via a switch so it runs only when you want it to.
If you ground the hot wire by mistake, I'd expect a loud pop, a blown fuse somewhere, and possible damage to things you didn't want to damage. So don't get into this if you're not confident in reading voltages and cutting and splicing wires (use an appropriate gauge wire).
I ran a lead from the ground wire to a switch on the side of the center console. From there it grounds at a screw that fastens the underside of the glove box to the dash frame. I kept the original ground path (via the hidden relays) intact and active. So even if I forget to switch the fan on, the computer will run it if things truly get hot.