no code needed for you, just wire it up
for 96-98 civics the keyless module was in the radio, so there are aftermarket wiring kits to keep the stock radio with power but wire in an aftermarket head unit, one of these kits would work perfect for you
or you can just hack apart the wires and do it yourself if you want to
splice and run power, ground, illumination, ignition and i think thats all you need
enjoy