There are some Decembers where the evenings are quite cold in SoCal. Last year -- early December (right around when you'll be there, gmi3804) -- it was 30 degrees at night. It was probably already 40 degrees by 6 p.m. or so, and then it dropped even lower.
So the heater/heat lamp question is a very good one.
One restaurant that I know of that has them outside is Rainforest Café in DTD. I thought that some of the other DTD restaurants (such as Jazz Kitchen) might have had them too.
I could have sworn that the BBQ in Frontierland has heat lamps, but maybe not.