[MetalMan]

It is a good idea to run a separate receiver power source instead of the BEC when using 12 cells. To disable the BEC, all you need to do is remove the red(center) wire from the ESC's receiver connector, and tape the loose lead onto the other wires.
With this setup, the switch on the 12012 will only act to turn on the 12012, but for it to receive signals the receiver pack needs to be plugged into the receiver.
You can use the stock Revo receiver battery. If it has a 3-pin type connector with only two wires, then you can plug it directly into the receiver. If you don't use a switch between the receiver and the receiver pack, then just make sure to unplug the receiver pack when you're done running the truck. If you do use a switch, then make sure to turn it on before you turn on the 12012's switch.