How to make led lights work off car electrical

Every year I put a wreath on my sprintervan.  I decided at the end of last XMAS to buy a plastic wreath and figure out a way to wire led lights into my sprinter electrical system so that when the head lights come on the wreath strapped to the front of the bus would come on.

Using a coupon I bought a string of qty 20 led lights that runs off a battery pack (qty 3 AA batteries).  It cycles through colors and patterns.

Tools and supplies needed:
1) string of battery powered LED lights
2) multimeter that can read voltage and current.
3) soldering iron with solder
4) some extra red and black wire
5) wire strippers/crimpers/wire cutters
6) shrink tube & heat gun
7) small connectors (male - female) type
8) resistors (you need to make a calculation to figure out what kind to use)
9) plastic wreath

http://www.mackconsult.com/?p=2272

Step 1 LED light string

Get a battery powered string of led lights.  They have to be led lights because strings of lights that plug in to a wall electrical socket runs on AC, led runs on DC.

Carefully disassemble the battery pack.  If you can save the batter pack this will help you later when you figure out what size resistor to use.  I then clipped off the battery pack (leaving the in line resistor already implemented). 

Then wind the light into the plastic wreath.