just did this on my 08' si coupe. was a piece of cake. i skipped the headliner completely and went straight for the fuse box. i also used fuse spot #8. I have a escort X50 and just used a spare phone cord i had laying around. my cord ended up being green positive, red ground.
theres a small screw by the panel under the dash next to the fuse box i used for a ground. worked perfect. then i just used a 20a fuse (all i have, will switch out for 7.5a later) and plugged it into the top spot of #8. detector comes on when the key is on, turns off when key is off. doesn't matter if the door opens, or anything else. heres a few pictures.
I mounted it to the left and low. hid the wire the best and its pretty stealth. not in the line of sight at all.
And here is the fuse box/ screw i used as a ground. sorry for the crap quality, but you get the idea
