* Kill switch: A toggle switch could act as a kill switch, cutting power to the ignition system or fuel pump to instantly stop the engine in an emergency or as an anti-theft measure. This is a common aftermarket modification for security or racing applications.
* Bypass switch (aftermarket modification): If someone has bypassed part of the factory electrical system (perhaps due to modifications or repairs), a toggle switch might be used to activate a modified starting circuit. This would be unusual and potentially dangerous unless done carefully.
* Part of a custom ignition system: In a heavily modified car with a custom ignition system, a toggle switch could be part of the starting sequence. This again would be far from standard.
In short, a toggle switch on a '91 Mustang GT to "start" it is highly unusual and likely indicates a non-stock electrical system. It's almost certainly a secondary switch for controlling engine power rather than directly starting the engine.
