regarding the led in tail lights

How many LEDs are you going to be using?  How are you currently planning on hooking them all up?

LEDs brightness are usually controlled in one of two ways - current limiting or pulse width modulation (PWM).

An ideal setup would run at 5vdc as to not be effected by battery fluctuations and would use PWM to control the intensity based on the parking light and brake light signals.  You can do it with current limiting but it will take some trial and error to get everything working perfect.  The "best" way to use current limiting would be to spec out a resistor for parking lights and a smaller resistor for brake lights.  Then, set the brake light signal up to interupt the parking light when it is active (so you don't have the added current of the parking lights going to the LEDs). 

No, 5vdc isn't too low at all, most LEDs are rated from 2 - 3.6vdc.  You just need to make sure your current and voltage supplies are designed correctly to achive optimal performance.

PWM involves sending pulsed voltage to the LEDs instead of contant voltage.  I recently did an LED dimming project that used a 1Khz pulse width.  The pulse is a square wave and can be adjusted from 0% (all off) to 100% (all on).  The cool thing about PWM is you set the system to work at full brightness and your intensity is then linear throughout the pulse width % (as opposed to the non linear results you will get from adjusting current). 

PWM will require a processor (or a controller) to use so it is a bit more involved.

Use 15 for parking and 25 for brake lights.

Do a google search for PWM you'll find lots of stuff.

All the PWM stuff I have done I have used a microcontroller.  Most microcontrollers will have the PWM hardware/software already integrated in to them so it is only a few lines of code and some interupts to make it work.