A sensor device for detecting whether a tractor and a trailer of a truck are connected includes a container having a plurality of tabs extending radially outwardly to engage an inside surface of a bearing plate of the trailer when the container is located in a hole in the bearing plate. A spring extends from a base of the container away from the face surface to bear upon an interior wall of the trailer to bias the tabs into contact with the inside wall when the container is located in the hole to maintain a position of the container therein. A sensor is also provided for sensing proximity of a ferrous material and being located in the container. The sensor senses proximity of ferrous material of a coupler carried by the tractor without sensing ferrous material of the bearing plate.