I believe this is what you're looking for.
http://www.autotoys.com/x/catalog/HONDA_ACCORD_20032004_T2__TOP_MOUNT_TURBO2_DAS
H_KIT_p_164.html

That will allow the installation of an aftermarket head unit.  Personally, I
just installed a set of subwoofers and an amp in the trunk.  As long as the
amp has "high level" inputs, you can tap the signal from the rear speakers.
Keep in mind that the factory unit does not have a 12v remote power on wire,
so any amp would have its remote on lead wires into the fusebox to a fuse
that turns on and off with the ignition.  That way, you amp turns on and off
with the car.  The downside to this, is that you can't really turn off the
stereo because the amp won't turn off and without any signal to the rear
speakers, they will microphones and send feedback into the amp and out the
speakers.  Believe, this sounds nasty with subs.  Of course, with an
upgraded stereo why would you ever turn it off?  If you want quiet, just
turn the volume to 0 and you won't have that problem.  I suppose you could
also install some sort of relay to turn off the amp when the stereo is off
but the guy who installed mine was an idiot and in the end, I decided to
live with this weakness just so that moron wouldn't touch my car again.

In the end, if you want a good aftermarket stereo, then that adapter at that
link is the product for you.