This is what you need:
1x Steering servo
If you have a gear box with more than one gear, then you need a servo to operate the gear box.
The servos normally get their power through the ESC. The ESC powers the radio receiver which has all servos connected to it. So power to the servos come from the battery through the ESC. In a crawler servos have to work hard and the amp draw of a steering servo is often more than a ESC can deliver. This is why most crawlers also need a BEC. A BEC is a small circuit that allows you to take power to your receiver directly from the battery. Most receivers can only handle 6V when batteries are 7.4V or 11.1V. This is where the BEC comes in as it lowers the voltage of the battery to 6V or below.
You basically have two ways of connecting a BEC to your system. Alternative 1 would look like this:
Here's is alternative 2 which will work equally well for your scale crawler:
This is the same wiring as above illustrated differently: