there's two DIN slots in the 96-98 civics. One near the HVAC controls, another by the cup holders.

to keep my OEM keyless, I installed my aftermarket HU in the lower DIN, and kept my factory HU in the upper DIN. You can pull the main wiring harness for the radio through the dash, so you don't need to extend it. From there, I wired the harness to the adapter harness, but I ran 4 extra wires from the Power, Ground, Ignition, and Dimmer, and ran them up to the stock HU. then I used some slip-on clamps (the smallest I could find), wrapped them in electric tape, and plugged them into the radio in the propper places (takes a little bit of looking at the main harness and figuring where wires should plug into). Keep the green harness plugged into the OEM radio, and you'll be set.

another thing to consider (though I haven't seen anyone do) is the 98-02 accords and CRV have a different keyless receiver (its close to 8"x5"x1", small demensions), and it'll accept the plug from the green harness that goes into the OEM radio. I noticed this when I installed the OEM keyless system into my father '00 accord lx v6.