Here is a response from Stephen Fagin who replaced Gary Mack:
Hello Mr. Smith,
Thank you for contacting The Sixth Floor Museum with a surprisingly challenging question. I did some research, reviewing about twenty years of newspaper articles that mentioned the sign, oral histories recorded by the Museum, and photographs of the building taken from every angle? and I am afraid I do not have a definitive answer for you. The Museum does have the metal faceplate of the sign, separated into about 128 individual pieces, and most of these components have loose wires on the reverse ? but with no indication of their original power source. The sign was installed in December 1959 and removed in May 1979, and as you know, the time and temperature function was discontinued in 1973.
The Dallas sign was one of only three Hertz billboard advertisements in the U.S. that had the electronic time and temperature apparatus, and despite numerous mentions in the news media, I was not able to determine ? from any news sources or Museum oral history ? where the power came from. I suspect it was not a battery, as it would have been virtually impossible in the late 1950s/early 1960s to power some 800 light bulbs on the sign with a battery without having to recharge or replace the unit every day or so. It may have had its own generator of some sort, though I have not seen any indication of this in any photograph and, from what I can gather, there was no generator on the roof when Dallas County engineers assessed the structural integrity of the building in 1978. It is possible a generator or independent power source was removed after the discontinuing the sign?s illumination in 1973, but it is equally possible that the sign drew its electricity from the building. I wish I had a definitive answer for you.