The "lighter" socket is just a direct line to 12V through an IGN switched relay at a 10A fuse. You can draw whatever current the socket allows, which apparently is 10A based on the fuse. It's not quite rocket science.

For 4 years I have been running an entire Pentium 166 computer as an MP3 player (
http://www.aaroncake.net/projects/mp3player.htm ) without a single issue. It draws about 4A. In the winter, I use a small inverter and string lights around the car AND use the MP3 player. Continuous draw is about 7-8A.
Keep in mind that the rear defroster grill pulls about 35-45A, so adding a little draw on the lighter socket is not going to hurt your DC-DC.