I found a picture of one of these on r3vlimited.com but there was no 'how to'.
You will need a small digital voltmeter, there are load of them on eBay for about £1-£2. They are described as 0.28 inches. Picture 1.
The switch disassembles using a screwdriver to push the tabs in. I threw away the rear part with the electrical terminals.
The front of the button just clips off, this is translucent red plastic and the voltmeter will display through this. You could use a heated window switch which is yellow/orange translucent.
I pulled all the copper parts and the bulb from the button, the white part also just slides out. Picture 2.
I removed enough material from the button to allow the meter to fit, making sure to keep both the sides on as that is where the front cover clips to. I removed just enough so the meter is held between the button and the cover and doesn't need to be glued in (this will make it easier to replace when it fails). I used a file, but a dremel would probably do a neater job. Picture 3.
I had to file the tabs off the side of the meter so it would fit.
Because I don't have the back of the button anymore, the front part is not held inside the body. I chose to screw a choc-block into the body so it presses up against and holds the button firm against it's stops, so it sits out (as if it were in the off position). I drilled a hole to allow a screw to hold the block in place. The choc-block also means that the delicate / cheap wires attached to the meter don't have to be handled when installing in the car. Picture 4.
To removed the hazard symbol, I used plastic polish, however, the origanl finish of the buttons is slightly textured and polish will removed this texture so it will look quite different. You could try to remove it some kind of solvent if you want to retain the texture. Picture 5.
Finished article installed picture 6
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