I saw the diagram for both units. The diagram for the HDS has the yellow going no where. The diagram for the structure scan has the black to negative, the red to a fuse then to positive, and the yellow to a switch then positive. If I am understanding you correctly, the yellow from the HDS connects to the yellow from the structure scan and when you power on the HDS, the structure scan comes on also. When you turn the HDS off the structure scan powers down.