I'm using this circuit for my front turn signals, but you can easily use this for brake lights, side marker lights, and just about any light on the car. Also, this can be used for wiring trailer tail lights.
(post some pictures of your set-up, if you want)
UPDATE: I recently made a trailer light controller using this circuit. It's a lot easier when you only need to make one trip to Radio Shack...
<IMG SRC="http://i1179.photobucket.com/albums/x388/irab88/CIMG7509.jpg" ALT="Trailer Light Controller">
(disclaimer: I am not responsible for anything you do. I will not pay your ticket if an officer pulls you over for a STREET LEGAL modification. It's your car. I will not pay your medical expenses (if you get shocked by the capacitor or burned with hot glue or...), as I already had to pay for mine. Be safe, be smart.)