You are talking about 7805 (12 V to 5V regulator)? If yes, then u might have cnfusion in using these type of regulatos.
These type type ICs are industrial horse in small power application
7805 regulator has fixed 5V output voltage and have input voltage range 7V ~35V. So it gives 5 V in the this input range. It has feedback system internally to keep voltage at constant 5 V.
Even if engine is off then battery voltage is about 11V ~12, but still this is in working range of this regulator.
At input 13-14 V, the out put is still 5V. So you don't need costly car mobile charger for powering this MP3 kit. that I am sure. Instead u can used 7805 (RS 10).
Have a look at attached file for input specification.
I have also same kind of DIY for poineer stock cassete player in my car from last 1 year. Still it is working perfectly.
Regards