Hi gizmo13, I'm kind of dealing with the same thing, except I'm using a 10 W 12.6 v 800 ma LED, the trouble you will probably run into is that the voltage on cars varies, you have ~ 12.6 without the engine, ~ 13+ after you start the engine and ~14.4 with the engine revved up. What I would suggest is use a current regulator made form a LM317, you can probably series 3 or 4 per LM317. Take a look at my post, you'll how I limited the current, trouble is, I had to have a higher voltage to make the regulator work, but I got it.
Kinarfi