We have discussed tail lights before, try a search. There are some nice pics.
LEDs are directional so need a diffuser in front or directed in different directions to spread out their light. Then their brightness is also spread out so you will need many LEDs.
Red LEDs are about 1.8V or 2V, so four can be in series for 8V, then a current-limiting resistor in series with them to limit their max current to about 25mA.
Another resistor can be in series with the entire array for reduced brightness as running lights. This resistor can be shorted with a transistor for full brightness for brakes and turn signals.