You don't actually drive a truck, do you?
There isn't a switch - the spring brakes (parking brakes) are actuated by pulling out on the tractor brake control valve. This evacuates air from the emergency chamber of the brake chambers, which in turn releases the spring brakes. The springs push the brakes into an engaged position, and that's your parking brake. It's typically the yellow knob (the red one is the trailer air supply). In the case of custom valve covers replacing the OEM ones, it'll be the one on the right.
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