As pointed out above, this is pretty much NOT a do-it-yourself project. Not if you want to keep it legal anyways. Vehicle lighting involves fairly stringent rules and design criteria that fills several regulatory books, more or less depending on what jurisdiction you are in. For starters, as metioned above, colour, angularity, lumens, retroreflectivity of the lens and housing including surface area, are strictly laid out and must be followed. This subject was brought up before here, and I posted a link to one of the manufacturers, where a design sheet was given. If you can read photometric charts, and are comfortable with calculus, search for them.
I do trafic control equipment and emegency vehicle lighting for a living, trust me, there are rules for good reason.
BTW, 99.9% of those aftermarket LED tailights you see on the little sports cars, the ones with the clear lens?, all are illegal. Legit ones can be had, but they are generally 2 or 3 hundred dollars apiece, since the R&D as well as approval testing by standards labs to get certification does not come cheap.