Unfortunately they product description is all but clear and there is no datasheet available. From the product description however we learn:
This simple on-off switch is rated for 20A at 12V [...] Note: The LED can be illuminated with as low as 3.3V.
To me that reads: connect anything between 3.3V and 12V and both the switch operates and the LED works fine. In my book that would include a current limiting resistor for the LED inside the switch, albeit one of the review suggests to have killed the LED while operating it without a resistor. Using 3.3V is also in the range of what the Pi's GPIO pins can handle. Yeah!
The review furthermore explains how to wire the switch as follows:
There are 3 connection points. Apply voltage (12V) to the +. Apply ground to the Ground symbol. The third pin is your output pin for when the toggle is active. These are generally used in automotive applications (12V) however, they can be used in other applications, hence the note about how low the LED will operate.
I measured the voltage drop across the LED as 1.9V
Combining all the information I'd suggest to do the following:
- Connect 3.3V via a 1.5kOhm resistor to the + pin of the switch. If the switch has no internal resistor for the LED this would limit the current to about 1mA (quite low for the LED to be on the safe side). The resistor should also not prevent the switch from properly toggling its output.
- Connect GND to the earth pin of the switch.
- Connect a voltmeter to the third pin of the switch (or a GPIO pin of the pin, if you dare).
If the output toggles according to the switch but the LED does not light up - decrease resistor. Again, I find it strange to believe that there is no appropriate resistor integrated but you're about to find out.