You don't have to remove the button you want to use but it does make it easier.
Insert a thin blade screwdiver in the side of the button, lever the clip in and push the button out.
There are 4 protruding tabs, file these down so they are flat.
Just like that (Tommy Cooper).
Reinsert the button and reassemble the button bank. Test the button works.
Of course it does.
Wiring The Button
Shown below are the 4 possible buttons you can use.
The number above relates the this wiring diagram.
The pin marked with a brown square is the earth, this is already present.
The pin marked with a red square is where you should fit your wired terminal.
If you look at the button bank, you can just see through the connection to the PCB below,
in the bottom left corner you will see a number 1 which relates to pin number 1.
The pins are counted from 1 (bottom left) to 9 (bottom right), 10 (top left) to 18 (top right).
The heated seat buttons use transistors on board so these are not ideal for using as a standard
switch. Stick with switches shown above which are straight through momentary action switches.
You will need to add a wire to the connector to use it, These just push in.
They are not easy to find unless you can find a scrapped smart.