I did the power in a friend's tsx. If you look at the battery from the front of the car and move your eyes up to the latch holding the hood on the driver side and then look about a foot below it and kind of to the right you should see a gromet that is accessible from the hood and the inside of the car. Just carefully poke through it with a screwdriver or something (i think that one has a couple of layers) and then push the wire through.
The trim along the doors is pretty much snap off, just pull them and run it under. When you get to the back seat, though, you have another little piece of trim that runs about a foot up the seat, you have to pull that completely out and then tuck the wires between the backrest and seat. Just bring the seat down, toss the wire in the trunk, and then, you're there!
Remote turn on is another story, if you want the amp to turn of with the head unit and not just the ignition or with whatever you grab remote turn-on from, you might need to pull your head unit out.
I couldn't tell you off hand if you could reach the factory amp (under the HU) without removing the whole head unit. Try removing those little compartments below the head unit and shine a little flash light in there and see if you can see the puny chunk of metal they call an amp (its down in there, not straight ahead). It has two wire harnesses coming right from behind it. If you can reach it, one of the harnesses will have a yellow w/ red stripe wire which is remote turn on for that amp. To be sure, put + from a voltmeter in the hole in the plug and ground your -. Turn on the car and make sure there is no voltage on that wire with the head unit turned off. Turn on the head unit and see if you get 12V. If so, BINGO!
If you are keeping ANY stock speakers or you are using the signal coming from the 6x9s, do not cut this wire, just jump off of it! If you are not going to keep stock speakers, you could just cut it. This also assumes that you are using the pre-amp signal, and not jumping off of any of the factory speakers (which it sounds like you are doing). Cutting this wire will disable all factory speakers including the signal going to the 6x9s!
Okay, one more thing. The signal coming from the 6x9s has a bass boost from the factory equipment. This means, if you have an amp with a bass boost at 40Hz and this system already has bass boost at 50Hz, you're gonna get some serious loudness (in a bad way) on only a few tones. I reccomend using the pre-amp outputs from directly behind the head unit (can't give you colors off hand). Just splice them into some shielded RCAs and ship them to the back and there won't be any equalization that could screw up what YOU specifically want it to sound like. Not to mention, grabbing the signal in the back will degrade quality as it is as this signal is already amplified. All you're gonna do is basically de-amplify and re-amplify.... hence the copy of a copy theory.
Another option is grabbing the preouts at the factory amp instead of removing the whole freakin' head unit but this allows the low-level signal to travel a good few feet unshielded. Fortunately, I don't think that would make any noticable difference in sound quality.
Actually, now that I think about it, that is what I would suggest because you want to mess around as least you can with that head unit. It has a bad history of loosing its illumination and never lighting back up again. Should this project void your warranty, I would hate to see you spend thousands on a new head unit for something that is the factory's fault. I think its better to keep you wire splicing as far from it as possible, just in case one day you have to argue with your dealer about whether the POS head unit is your problem or theirs.
Just be sure that if you plan on using some of your factory speakers, DO NOT COMPLETELY CUT ANYTHING! Just jump off of it (a little stripping, a little solder, a little tape). This includes the pre-amp signal. For example, if you use the rear pre-amp signal, which runs the doors and 6x9s, and all you remove is the 6x9s, you still need signal for the rear doors so it must continue through the factory amp as well as the sub amp.
Also, I won't yell at you for taking the easy way with the 6x9 signal, many audio shops have done that and had good results.... but not all audio shops have people with a whole lot of experience (I consider experiece knowing how to apply knowledge from MANY subjects to one application). And not all employees at audio stores are true audiophiles.