I have never re pinned a fuse box before and it isnt hard, just get the pros to do it.
http://www.talkford.com/community/topic/183707-cigarette-lighter-permanently-live/
As for the relay, You could fit it all in behind the dash and only have to put it around the 12 socket, Run about 50cm of sensor wire to your ignition feed where ever you decide to take it from. Probably the radio.
You would only need to splice the wire, You can also get relay connector blocks and put some pins in so its all tidy.