In my view you have a couple options:
- Hack up the cig adapter your set comes w/ and solder or wire-tap it into the accessory line behind the dash
- Buy another cig adapter to hack up, thus keeping the one that comes w/ your set intact
- Buy a cig adapter splitter and hack that up, wire one of the cig adapter sockets into the harness behind the dash, then plug the cig adapter into it (and tie wrap it in place) behind the dash, then close everything backup
I went w/ option #2. Radio-smack sells universal adapters w/ bare wires on the other end. Add a fuse just to be safe. This was a real must-do in the miata as the interior is too nice looking to have extra wires everywhere. I've been toying w/ doing #3 w/ a belkin adapter I bought for my ipod now. It has a mini amp and headphone jack built in, so I can't just lob off the cig-adapter end on that one.
Right now I have a line-in cable coming out from the glovebox (I've since given up on the FM modulator). Now I need to figure out a way to deal w/ the line in cord. Cord/cable routing is much easier in the truck w/ the way the radio/HVAC bezel works.