I wanted to do the same thing you did (make your own LED tail lights) because I saw on www.cardomain.com
a guy made really cool custom LED tail lights for a third gen camaro ( http://www.cardomain.com/memberpage/274135
Look about half way down the page ) I'm still working on mine because I don't have a whole lot of free time and I have the stock tail lights anyways, so it's not a big rush to get mine done. What I finally did was go to http://www.myra-simon.com/bike/LEDlight.html
and looked at these. These are for bicycles so obviously yours will be a little different, but she gives a lot of good information on making them and what volt resistors and what you will need for how many lights, and the source voltage and what not. Hope this helps you out.
AIM Name: funrl fr a frnd