I don't think you need to use both sides of the switch for switching sources as they would share a common terminal, you just need to switch the other line. I use the second side of the switch to switch the power to the BT module.
You will need 5V-35V DC, so you will probably need a rectifier, smoother and a 5V regulator. At least, that's what I did.
I should really draw circuit diagrams when I do these things!
I used
this BT module.