Disconnect both connections to the ballast resistor.
Turn the key to the "on" position.
Take a test light with the clamp on a good ground, and poke the other end in the ballast resistor connectors (while they are loose). The one that makes the light go on is the side that you want to tap into for your 12 V source. DO NOT USE THE COIL FOR A POWER SOURCE.
I like to use a piggy pack terminal (you can get them on the electrical rack at NAPA) and then stack the terminals. You can then remove it without any trace later if needed.