Yes, you should always check for power to the lamp first. Did the origional lamp work? I was assuming that it did. If you don't have power to the wires when turned on, do one more check, connect a long wire to the negitive battery post and to your test lamp. Now test for power at the rear lamp. If you have power now you have a ground problem from the battery to the frame.
Lots of people hook up the negative battery cable to an engine or transmission, that is ok but you also need a cable from the engine or transmission to the frame (battery negative to frame). The reason for this is that the engine and transmission are isolated from the frame with rubber mounts, and they may of may not ground. Some ground through the carb linkage or fuel line (if it is metal) if a ground directly to the frame is not attached.
