Wemooz,
I have to give you another option because a single switch like that just cannot handle so much current very long. It's doomed from the start.
I have used about 6 different setups in 25 years. The best setup is made with one of these:
Hayden Automotive 3653 - Hayden Electric Fan Controls - Overview - SummitRacing.com and two of these:
(5)12V BOSCH STYLE RELAYS 40A SPDT + (5) 5-WIRE SOCKETS | eBay
Find a small plastic box and mount the thermal switch in it with the shaft sticking out of a hole. Mount the relays in there too. (I use two for redundancy and because they're cheap and light.) Then, put a knob on it and mount the box as high as you can in the engine compartment. Very carefully unroll the copper tube and slide the probe into your radiator fins near the water outlet (you only want the fan to come on when the radiator is not cooling the water). Use plastic split wire loom to protect the copper tube like this:
100' BLACK PLASTIC SPLIT WIRE CABLE LOOM - 3/8" | eBay and use cable ties to run it along safe areas.
There are several advantages with this setup: 1. You can choose the temperature for the fan to come on (I like 200) . 2. You can connect a low current (cheap) extra switch in the dash to turn the fan on. 3. You can connect switched power so the fan goes off when you turn the engine off (prevents dead batteries). 4. If you do 2, then you can easily add a light in the dash so you know when the fan is on. 5. You don't have to remove the old switch (just pull the connectors off to connect to the new system). 6. You don't have to drain the cooling system.
This setup has worked very well for me for many years. If you have any trouble with the wiring diagram, let me know.
RS