Here's what I do...
* I leave my phone's Bluetooth turned on all the time. (The power drain is completely negligible since Bluetooth goes into a low-power, periodic-sniffing state when not actively communicating. In 24 hours, it's probably the equivalent power drain as leaving your screen on for a few seconds. I doubt you could tell the difference.)
* In the phone's Bluetooth settings, long-press the car stereo name and set its "Connection Access" to "Always Connect".
Then the phone will always connect when it "sees" the car stereo and will disconnect when the car stereo is off.
More recently, I've been using the
free Llama app to do more advanced "if-then" things.
* Example 1: If my phone "sees" my car stereo's Bluetooth signal, then I have it maximize the media volume and launch Pandora (or every few months I change it to launch a playlist with my favorite music player; Mixzing).
* Example 2: If my phone is charging and it "sees" my car stereo's Bluetooth signal, then I have it turn up the media volume, turn off the screen timeout, maximize screen brightness, launch Pandora, and launch Waze (navigation).
In both examples, I have Llama turn everything back to normal when the car stereo is no longer "seen".
Llama can do all kinds of other useful things and it uses a negligible amount of power. You can teach it to do stuff at certain locations based its recognition of the available cell towers. That kind of location detection uses virtually no power, as opposed to GPS, which gobbles power. For example you can teach Llama the location of your church, school, and/or movie-theatre, and have Llama turn off the ringer when in those locations, but turn it back up when not in those locations.
