It might be better (just my two penny worth) to wire the supply to this in through a seperate, fused socket.
Two reasons for this:
1. I've never found the cigar lighters on Seis or Cinqs to give reliable power to anything (be it mp3 player, lead light, navigator) when attatched by a flexible lead: any vibration, bump, etc. is liable to break the connection.
2. Modern cigar lighters only work when the key is in place and turned because of a number of accidents involving kids playing with cigar lighters, some of which, I believe, were fatal.