Partially to share with others, and partially so I can find the dang link when I need it next, here is the procedure for rewiring holiday lights to run on 12vdc.
I did this with a string of lights last winter to have on my cargo bike. They looked awesome, but I would lose another few bulbs every time I hit a big bump, it seemed, until the string was about 2/3 dead. I completely missed the (traditional) holiday season this winter, but I've got a string of LED lights to convert in the same fashion. I figure they'll be far more durable and draw less current. My usual power source is a 10 amp-hour sealed lead acid battery, as would be found in an uninterruptable power supply or an exit sign. The incandescent string drew around 3 amps if I recall correctly.