LED's are wired in parallel. You can hook almost an infinite number of them together as they draw very little current. Most all LED's for automotive use are rated for 12vdc. If you choose LED's of lower voltage rating you must drop the supply voltage down to their requirements. There are also hi intensity LEDs that are perfect for automotive use. As far as I am aware of they are available in clear, red, green, blue, and yellow. Here sre some sites that carry LEDs.
Digi Key Electronics
Both sites do not have minimum order requirement and both have excellent web sites and paper catalogs.