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If you open the hood you'll see what MarcusF means.
It is drive by wire, it's just not your typical modern setup. The sensor is not in/around the gas pedal itself, it's in the engine compartment. The gas pedal pulls on a metal wire, which then drives the sensor. You can find it next to your brake fluid reservoir, here's a random picture of it with the throttle wire on the lower side:
Mercedes SLR McLaren 722 S Is Extremely Rare Example Modified by McLaren
Slideshow: A one-of-one U.S.-spec Mercedes-Benz SLR McLaren Roadster became even rarer after a factory-backed transformation at McLaren's headquarters.
