Don't know your exact electric panel, but assume it is similar, if not identical, to mine (2004 B36) - since the lighter socket is part of the panel the supply for it goes directly from the switch/fuse which is also on the panel, so there is no dedicated terminal.
So you options are either splicing or, as suggested, use one of the free circuits.
One note - do you need them for 12V or as a socket for charging USB type devices? If the latter then you can purchase sockets that are similar in shape and size to the cigarette lighter but have on them 2 X USB charging ports (I purchased several of them on Ebay)