The extra cooler is great, and it's good to hear the fluid is not overheating. Now for the project. The following are images of what I hope is your vehicle, they're from a 2003 USA Montero shop manual. The first shows the transmission with the harness, and the first connector [C-03] that the TCC circuit passes through. This probably isn't an ideal place to access it, but in cases where the interior of the vehicle has been customized sometimes it's prudent not to disturb this work. This image also shows the transfer case which you may or may not have.
The TCC circuit is pin 7 in this connector, and the wire color is Yellow with a Red stripe. The color remains the same all the way to the PCM, which is the next image along with the connector [D-136].
If you pull off the lower kick panel over on the passenger side, you should be able to access this connector. Pin number 130 should be the same Yellow/Red wire. The internal wiring is the next image, and it identifies the TCC solenoid so we can tell which wire it's controlled by.
The TCC is the fourth from the left. You can also see the huge connector [E-114] that passes the electrical connections from the whole harness, but it's not in an easily access place nor is it an easy connector to work with>too many tightly packed wires. I would splice into the wire about six inches [15cm] or so from the PCM. This is not a high current circuit, and inside the vehicle behind the kick panel is an area that usually stays dry, so a Scotch-Lock splice would work well here. They look like this:
This has two channels; one that goes all the way through and one that's blocked at one end. The existing wire slides into the connector from the side, the unstripped wire that will go to the switch goes into the blocked passage. Next the metal blade connector is pressed into the plastic body, and as it goes in it slices through the wire's insulation and makes a connection between the two wires. Lastly, the cover snaps over the blade as insulation. I'd wrap it with electrical tape to be safe. Mount the switch [a simple on/off type] anywhere that's convenient. You can, but don't have to, use a 5 amp in-line fuse on either wire to switch. Connect the other wire from the switch to any good ground, some locations are in the next image.
Ground point 12 is just below the PCM, and it would be easiest to run the two wires from the switch to points that are near each other.
Now, with the switch in the "open" or "off" position, the PCM will control the TCC. In the other position it will lock the clutch at 100%. The PCM does not normally do this, it controls the TCC solenoid using brief connections to ground that vary in on-time duration-this is called pulse width modulation. If the PCM detects that the TCC remains locked for or ten seconds longer than it should be it will probably set a fault code 52 or 53, but there's no fail-safe strategy indicated in the diagnostics for these codes. Other codes that may set could include the gear ratio error codes for the forward gears 41-45. The ratio error codes have the same failure strategy as the 36 mentioned earlier.
That should be all you need to complete your desired task. If you have problems with the PCM setting codes and becoming stuck in 3rd gear, there is a way to control each solenoid individually with a switch. You can purchase a 'transmission test box' or make your own. The solenoids are controlled singly or in groups to obtain all the forward gears and reverse. In either case it turns the automatic into a manual without a clutch pedal. Test devices like this are used to diagnose transmission problems or to control the transmission for better performance such as in racing. Let me know how you make out, and good luck.