this how to do was copied from another forum, but have done the same and it works great.
I ran the #7 fuse from the trunk. Much easier (IMHO). Here's a blow by blow:
1) open rear trunk area covering by peeling back the gray liner. You can skip this but it's easier to fish with it out of the way.
2) pull up the strips along the floor of both driver side doors (or one door if coupe).
3) use electicians fish-tape or heavy gauge wire to push through from the trunk area to the floor area near seats in the rear. This is simpler than it sounds.
4) pull wire from the door area back to the trunk and leave enough to terminate later.
5) tuck wire into wiring duct. This duct is about 1" thick by 4" wide and is located about 1-2" past the edge of the carpeting. Peel back the floor carpet (you remembered to take the threshold strip off in # 2 right?) and you'll see it. It works like a taco - seperate it by popping the snaps and you'll see the wiring looms.
Run your wire along side the others all the way to the front.
6) when you get to the a-pillar foot well area, run the wire up through the kick-plate trim and bring it up to about the dash area.
7) if you are using a V1 terminate your location here. Pop off the cabin fuse panel cover, run the wires you just pulled into the dashboard cavity. Terminate your power distribution block (v1 block) with your V1 fuse on the newly pulled wires and insert into the cavity behind the cabin fuse panel.
If you are running another brand of detector run the power as follows...
8) take your power phone cable and plug into the distribution block.
9) pop off a-pillar cover. This thing is pressure snapped on - so gentle force straight toward you will get it off. You can run your wire parallel to the cable that's in the a-pillar. This is the cable for deploying the air curtain, so be nice to it! Take your phone wire (or your continued cable from below for non V1) and run it to the headliner.
10) run the cable through the headliner by gently pushing it in from the front. Nothing additional will be required to keep it in place - so don't sweat it.
11) all set? good, go back to the trunk. Terminate your hot wire to #7 - I went to the hardware store and got a spade with an inline fuse holder (I recommend this path). Also, take your negative cable and terminate it with a round or spade end. Attach this to the ground post. On the coupe it was located under the rear taillights and on the sedan it was to the right of the fuse block. Hard to miss really, it has all the brown wires coming into it.
All said and done this took me about 30 minutes the first time and about 15 the second time. It seems the right (not hack way) to go about it and I learned all about the electrical routing as part of the deal.
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