What kind of wire did you use?
Also, I couldn't tell from the pictures, but if your sensor wire is not zip-tied to the car somewhere, it's a good idea to do that.
I used Tekin 12g for the motor and Deans 12g for the battery. I used the hold down clip for the sensor wire on the back of the motor. Plus it's attached to the body post with a piece of velcro tape. It won't fall out.