"The important thing is the 2 port vacuum switch...you need to get one that is 'open' when it's in the off position so vacuum can flow through normally. I used a Nippondenso EGR vacuum switch for a Honda Civic. They never break! The rest of the parts list is simple and small:
Rocker switch (I got a normal non-lighted black one from Radio Shack),a 5 amp inline fuse holder,1 foot of vacuum hose and enough electrical wire to go from the vacuum switch to the on/off switch.
My Y2K truck is a manual trans so I chose to put the on/off switch on the floor console between the t-case shift lever and the big cup holder.
I tapped into the "power outlet" wiring for power and put the inline fuse holder between the power outlet wiring and on/off switch.
I then ran a long wire from the on/off switch out to the engine compartment.
I carefully ran this wire along with the factory underdash wire harness and
zip-tied it as I went along so it looks clean.
On my truck I mounted the vacuum switch to the metal bracket above the intake manifold that all the wire harnesses attach to. I removed the bracket,drilled a mounting hole for the vacuum switch,and reinstalled it. The vacuum switch has a 'pos' and 'neg' terminal and I used a short piece of wire to ground the 'neg' terminal to the same bolt that I used to mount the vacuum switch to the bracket. Now the wire that I ran under the dash,into the engine compartment is connected to the 'pos' terminal in the vacuum switch.
Right above the bracket,you'll see a vacuum tee with two ports...one port is capped and the other is the line that goes down to the transfer case. Disconnect the tranfer case line from the tee and plug it to one side
of the vacuum switch and cut a piece of vacuum hose and connect the other side of the vacuum switch to the tee...and you're all done! Simple,eh.
The only thing I might recommend is a different place to mount the vacuum switch. I mounted it to that bracket because I wanted it as close to the tee
as possible...bad idea if you're running Isspro gauges like me. That bracket is holding about 50 wires,my EGT probe wire and that vacuum switch...well there is so much RF (frequencies) going on there that once in a great while the vacuum switch amplifies a signal and affects the EGT gauge reading for a minute. I still need to move my vacuum switch to a different location.