Although you can get a plug for the radio, [reverse harness] even an adapter harness for the key-less entry plug, [car side], I know of no other suppler other then Honda that would have a reverse harness for the key-less entry port on the stock HU.
I assume you have the the green key-less plug but "5 pins are missing".
Do you mean 5 pins have been depined from the plug and they are still on the ends of the wires and all you need to know is where they go?
Or do you mean some DA cut the leads so close to the pins in the plug that you can't tell what color/tracer was on that pin?
Either way all my Haynes Manual gives me is the color code, no pin-out and in their great wisdom, nothing more, so I can't be of any help.
I did a little Googleing but could not find a pin-out, but I am sure there is one.
You can always hit the bone yard and get lucky and find a 98 Civic with damage to the dash harness so they will not charge you for the whole dash harness even if all you take are the radio and key-less entry plugs, [W/ 5-6 inches of wire].
Making it a color for color, solder and shrink tube, plug and play.
Maybe someone could take a pix, [good resolution and color] of the plugs, wire end.