Forgive my ignorance here but if I'm not mistaken the 7805 regulator is always powered regardless of the vehicles on or off status and the 50 OHM resistor will drain 100mA all by it's lonesome. Add the draw of the rest of the circuit and the draw of the RFID reader (25mA avg, I know because I own the same make and model) and you're over budget on standby current. Take a day off driving and you'll likely have a dead battery. I contended with the same issue for a similar project and current draw is paramount, it's needs to be kept in check for reliable use.