I hooked up my radar detector permanently today. I just wanted to post some notes on it in case anyone else has been thinking about doing this.
Before you do anything, engage your emergency brake (especially on automatics if you guys don't usually engage this)
The detector is a Whistler model 1440. Pretty run of the mill unit. It comes with two brackets, one for the windshield mount and the other for the sun visor.
Take the visor mount and, with the detector mounted to it, clip it onto the headliner somewhere between the two sun visors. I did it between the visors because of visibility issues (I'm 6'1") and clearance issues (I'd like to be able to open the visor). In my case, I mounted it just to the right edge of the rearview mirror.
Get a spool of wire ready. I used 14 gauge wire myself, as this should easily be able to handle the power requirements of the detector.
I took the cord that came with my detector and chopped off the end with the small plug on it that goes to the detector. I maintained about four inches of this wire. Then I took my 14ga wire and spliced it into the plug wire. Make note of positive and negative before you go any further. You'll find that there is a space between the headliner and the roofline that is more than ample for stuffing the wiring up there. Run it over to the closest A-pillar.
I chose to run the wiring down the passenger's side of the car. Grab a hold of the A-pillar molding and gently but firmly start pulling on it. It clips into place and will just pop out when you pull hard enough. You will notice at the bottom of the A-pillar that there is a hole leading into the cavity behind the dash. Fish the wire around in there until you can pull it out from the bottom. Once you can pull this off, get the wire set into place nice and tight and snap the molding back into place.
Get yourself some wire ties and tie the wiring off to anchor points behind the dash to keep it safe and out of eyesight. You'll get just about to the transmission hump and STOP THERE.
Put the key in the ignition, in the running position (but don't start the car) and pull the shifter back to 1 if you have an automatic. If you have a manual transmission you can move the shifter without putting the key in. With your foot on the brake, pull up on the plastic trim around the shifter and the whole piece will come up. Don't pull too hard as you have the accessory socket there and still attached!!!!
Pull off the plug going to the accessory outlet. I chose to use this wire because it is fused and should provide ample power for the detector plus any other accessory that I'd ever use. The downside is that this wire is ALWAYS HOT so you may want to find a wire switched by the ignition position. Your call.
Anyway I got some line splicers from Radio Shack that are made to go in-line. Run your wire back behind the center of the dash and pull it through by the shifter. Cut to length but give yourself a good 4" of play to work with. Strip no more than 1/4" from the ends of the detector wires. Put them into the one-way hole in the splice. Put the slotted side of the splice over the insulated wire going to the accessory socket (make note of polarity!!!) and squeeze with a set of pliers. Repeat for the other wire.
Put everything back together in reverse order, and plug the radar detector in. You should immediately hear the joyful sounds of a working radar detector (unless you picked a different hot wire in which case you'll need to turn on the key).
The installation is very clean looking, and all of the wiring is out of the way. I usually stash my detector when I park anyway so it doesn't bother me to be tied into a constant hot wire.