Disclaimer: I am not responsible for the death of your Xbox360 or your automobile if this project goes a rye. There is slim chance that anything could happen but be careful and when it doubt use a ohmmeter to ensure the correct volts and amps. GOOD LUCK!
Step 1: Material List
1 Xbox360 with power cord ($300.00 and up)
1 Roadmaster Vrfm8 Fm transmitter. about ($20.00)
1 Female 9 Position interlocking connector (Radioshack.com part number 274-229) $2.59
2 Male 9 Position interlocking connectors (Radioshack.com part number 274-239) $2.59
1 Three pack of wire at Radio Shack (18-20 AWG, 1 red, 1 black, and 1 green spool) ???
Also: Electrical tape, soldering gun or iron, solder, and I like using a third hand that hold objects to make soldering that much easier.