This is very easy to do. I just did this install with PIAA fog lights and the OEM switch.
Give me a second to post up some information.
First off, if you want the OEM button and Harness, you need to order one from a dealership. I got mine at Toyota of Dallas.
FOG LIGHT SWITCH / HARNESS PT297-35070-SH-AS
Next, I utilized the OEM instructions on routing the wires in the engine bay and within the cab. It is very helpful in taking apart your lower dash components and console.
http://lastgreatroadtrip.com/myFiles/litefj71AUX.pdf
I assume your KCs are pre-wired the same as my PIAAs. You should be able to wire everything from the Relay (provided by KC) to the lights as per instructions. (see page 4 of linked PDF for wiring diagram)
http://www.piaa.com/Lamps/install/520.pdf
From the relay you should have wires running to the KC switch. (on my PIAAs I had 2 from the relay...power and ground, and one more coming off the PIAA switch to go to a 12v power source.)
Cut the KC switch off, and discard the loose third wire and KC switch. Now you should have only 2 wires coming from the relay to route through the firewall and into the cab. you can route them through the rubber boot at the drivers side under the hood hinge.
now you are ready to wire the OEM switch. See link for wiring info. this is what I used.
http://www.fjcruiserforums.com/forums/how-technical-articles/49978-oem-aux-light-switch-diagram-schematic.html You will need to cut the connector at the non-switch end of the OEM harness off.
I routed the harness and grounded it as shown in the OEM instructions (link above). This put me right under the fuse panel to come up and plug my switch power into an add-a-circuit (pick one up at any auto store)...
more on this below. It also puts you close to the rubber boot in the firewall to tie in the power and ground to the relay.
You will have to choose a power source for the switch. The Relay (under the hood) power is directly tied to the battery the way they come. Now you need to choose power for the switch. It can be constant/ key switched/ or tied into High beams or low beams (so that it can only be on with that light.)
I chose to use an "add-a-circuit" in a 'key-switched' blank inside fuse panel (or you could use an aftermarket fuse panel as some do for many accessories.) I chose this because I don't want them to stay on when the key is turned off.
See picture below i grabbed from another thread:
"This picture came from another thread from our esteemed colleague SwissArmy.
The blue shows switched fuses and the black shows always hot fuses. You can see in the picture he has used several of them and I believe at least some of these were for fog lights."
my finished product...