Engine fan switch on cat 6 NZ

ok. Before I bought this truck 4 years ago some idiot has went and ran all kinds extra wires to the extra switches ,
Now I have a cut off engine switch on one of my switches , if the switch is in down position it will try to start but won't start. It has to be up to start. So I have a train horn and the push button switch went bad so I opened up panel where all the extra switches are and I noticed some wires went to nothing they were hooked up to switches but was cut at other end. So I took the wires off. Now there were 2 wires just stuck down in one of the switches for the engine fan to manually turn it on. So I took them out put an end on them and stuck them back where they were. Now that I did that all of a sudden I manually turn my fan on and my truck dies.
I didn't not move any wires. This wiring is so messed up. Now what I need is how can I run a separate switch myself with out even messing with the ones already in my truck to be able to manually turn my engine fan on and off if I want to. It cuts on automatically on its own. But as when it gets hot outside it gets up to 200 Degrees before it cuts on and only way to keep my truck ac cool at that degrees is turn on and run engine fan while I sit and idle truck at 1000 rpm .
Where would the wire be going to the fan so I can do that on a separate switch I install myself. And just leave the other switch alone. Cause aparentaly it has to stay in down position to keep the truck running now all of a sudden.
So what I'm asking is I need to install a manual fan switch and need to know where to find the wire to do it from the engine fan.
Cat 6 NZ 475 single turbo.