UPDATE: I found this on a custom car stereo site. Everyone's been asking, so here is the circuit. I still use a battery! "Build the circuit on the diagram to step down the voltage from the vehicle's 12 volts (the circuit works for up to 35 volts) to 9 volts. Make sure that the battery plug is soldered BACKWARDS to the circuit (black to +12v ignition and red to ground). Once the circuit is built, hook it up to the car's ignition (key on) output and test output voltage and polarity with a voltmeter. If the voltage is 9 volts and polarity is correct, plug up the voltage regulation circuit to the remote's battery terminal. Test to make sure everything works. Wrap up the circuit in electrical tape or heat shrink to avoid shorts."