Originally Posted by BlueGlowBoy

Depending on which BEC you have, and how much current it supplies, you should be able to move it from your 2wd to your 4wd.

No, you cannot splice it into the speed control cable that plugs into the receiver. If you power your receiver from both the speed control and the BEC, then you will likely damage your receiver, and possibly your servo and speed control (and transponder).

You can do this two different ways that I can think of.
a) remove the red wire from your servo plug and wire that to the BEC. This will power your servo directly from the external BEC. The ESC can still power the receiver and transponder.
b) remove the red wire from the ESC's receiver connector. This will prevent the ESC from powering the RX. Make a connector for the BEC that uses a regular RX plug, but only uses the red and black wires. Plug the BEC into any empty channel on the RX. many RXs even have a spot that is specifically for the BEC. This will power the RX, Servo, Transponder, and any extra stuff you have that would normally pull power from the RX connection.