A lamp support assembly, retrofit kit, and method are provided for lighting a sign from an interior thereof. The assembly can be used as a drop-in replacement for high-voltage gas-discharge tube lamps such as high-output fluorescent lamps, and may be used to retrofit signs originally built for gas-discharge tube lamps with low-voltage, high-efficiency lighting such as LED lighting. The assembly includes an elongate support member for supporting electric lamp units, with end caps at opposite ends of the support member for engaging the original electro-mechanical mounts (or replacement mounts) in the sign. Thus, the lamp support assembly can be supported in the sign in a similar manner as gas-discharge tube lamps, and can be powered by a low-voltage power source, which may replace a higher-voltage power source in the case of a retrofit.