My advice would be to book something now, but don't prepay. I'd rent from one of the on-airport agencies: partial list is
Alamo,
Budget,
Enterprise,
Avis, National, Dollar, Hertz, Firefly. I sometimes use some of the generic sites (like Kayak, carrentalsavers, Orbitz) to do research, but I prefer to do the actual booking at the agency site.
The good news is you're going in January, so crowds should be down, as well as car rental demand. You should find good rates.
But, here's the most important part. Car rental rates tend to be their lowest only as close as two days before departure up to about 3 weeks. You're not going to find the best rate now...but I would cover myself with a non-binding reservation now, just in case...and especially if you are renting an unusual car class.
Continue to check occasionally if rates have gone down (they may go up...car rates are more volatile than airlines), but especially check frequently as the trip gets closer. There is no harm in rebooking, but please remember to cancel any reservations that you aren't going to use.