I have an 02 Accord, so hope to be of assistance, although I think they changed to a new design with the 03 model year.
For the power cable... there should be a plastic knockout block a ways above the gas pedal, just under the cowl panel. You should be able to see the knockout block under the hood by inspecting the fire wall carefully. You can take it out, cut out the center part of it, replace it back in the firewall, and then run your power wire through it. You can add split-loom tubing for the wire run under the hood if you want it to look nicer. If your Honda is anything like mine, you WILL have a nasty noise in the vehicle's power source. I seemed to eliminate the noise by placing ferrite cores on both leads of the power cable and on the alternator wires. I also picked up Radio Shacks 20Amp noise filter, which is mounted to the top of my radio mounting plate (see below). I ran a 1/2" ground strap from my radio to a ground point near the center console area, but am not sure if I should have done that as I blew the airbag modole when I transmitted with 75W for the first time.
For the radio... the empty area in the center console (underneath the AM/FM radio and climate controls) works great. You can remove the diagonally placed trim piece (by pulling it firmly straight down), then make a custom metal plate to mount your radio to. Drill two holes in the edge of the plate closest to you - remove the two Honda trim screws, place the plate in place, and then replace the trim screws, holding the plate in place. It's hard to describe.
For the antenna... I did an NMO roofmount. I used a 3/4" Greenlee punch to make the hole and am satisfied with the results. The headliner comes down easily if you take your time - if you go that route email me and I can provide you the information on how to do that. I routed my coax down the passenger-side windshield pillar to the underside of the dash, then to the back of the radio mounted in the center consol area. Another idea for you would be to use a trunk-mount antenna and route the coax to the front of the car, or get a radio with remote-face capabilities and simply mount the radio in the trunk. If you want to get fancy, you can route the cables under the rear seat, then under the rear carpet, then under the console, then to the center console area. Again, if you take time doing this, the rear seat and center console can be removed with minimal effort.
Let me know if you have any other ?s.
Dan / ab0re