Well, even if you could get a keypad to work and lock your keys in your car, you would have to find a way to disable the proximity button. If you didnt the proximity button would still work because the keys are in the car and anybody could open it.
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The system has some ability to identify whether the key is inside or outside the vehicle.
For security reasons, I suspect that it disables external unlock when the key is detected inside the vehicle.
Otherwise, anyone would be able to walk up to your vehicle, press the button and gain entry; a carjacker's delight.
Can you test this before ignition, ignition on, then ignition off? It has been a bit since I read the manual, but if recalling correctly, there was some bit about the system disabling any key that has been left inside of the vehicle for a period.