* Factory Alarm: If it's a factory system, the module is likely tucked away somewhere under the dash, possibly behind the glove box or in the center console area. It might be difficult to locate without a wiring diagram specific to your Ranger's build. You'd likely need a service manual or a detailed wiring diagram to pinpoint it.
* Aftermarket Alarm: If it's an aftermarket system, its location is entirely up to the installer. It could be virtually anywhere—under the dash, in the engine compartment (less likely due to weather exposure), or even in the trunk. There's no single answer without knowing the specific brand and installation.
In short, without more information about the specific alarm system in your 1999 Ford Ranger, it's impossible to give a precise location. Checking service manuals (available online or at auto parts stores) or contacting a Ford dealership might be helpful if it's a factory system. If it's aftermarket, carefully inspecting common hiding places under the dash would be your best bet, but be cautious not to damage any wiring.
