I had an original Rally Pac on a 66 Fastback that drained the battery, actually several, while it sat. It was the clock that did it. Let's say that I have several Mustangs (and other vehicles too) so daily driving was not practical. On another car I had a repro crystal controlled clock in the Rally Pac and had no problem with the battery, and it kept good time too. I have two Mustangs in my garage right now with original Rally Pacs, and I use battery disconnect switches on both because the clocks kill the batteries, if you don't keep them charged, as in trickle. Just a bit of info.
Jim