If you are going to leave the car for more than a few days, then simply disconnect the adaptor from the ipod (just squeeze the plug on each side and it releases. Li+ ion batteries will fail if they get too hot or too cold so its best not to leave the iPod in the car on hot days (its not like you are going to encounter the deep freeze in FL....).
BTW, the power draw on the car's battery is almost insignificant but you could also go into the Settings menu on the iPod and set the backlight timer to "off". I also bought an 80 Gb iPod for the car. That's a lotta songs
