I think this is what I'm looking for. So here's a question or two for you. Is this worth it? Will my battery die if I leave a fm transmitter or a bluetooth adapter plugged in all the time? If so, how long? A day? A week? A month?
Whether or not it is worth it is up to you. It is a fairly easy conversion, whether you use a relay or take the power directly from the RAP connection.
Bluetooth or FM transmitters are not going to have a significant impact on how quickly the battery runs down. Other parts of your car constantly have power - the ECM, the stereo, the BCM, the keyless remote receiver, and so on. Left alone for a month, your car battery would probably go flat just from the drain of these parts - so again, leaving one powered all the time won't have a big impact.
I tied my GPS unit into the RAP supply when I had the dash torn apart for other reasons, but prior to doing that I just left it on all the time, no problems. If you have a similar accessory that will always stay in the car, you might want to hide it behind the BCM access panel - especially if you have a bluetooth receiver/FM transmitter combo.
~ MattInSoCal