I am making my own sequential circuit from a schematic posted earlier. I took the tail lights out and noticed that the inside light doesn't light for signaling, just braking. There is no wire to the second filament for signaling. I don't know how the plug n play or splice in kits light the inside lamp, must use the brake filament. The bulb holder doesn't have a wire to the signal filament for the inside bulb, so I drilled thru the sealed unit and soldered in a wire. I've spent about $25 for the parts and wire. So I've saved money and have the satisfaction of building it myself.