When I got a new headunit in 2000 for my 99 civic, the correct harness wasnt availiable, so I had to just splice up everything and make sense of it from the manuals and trial and error with a voltage tester. I would suggest that if you can find a harness use it, if not get a voltage tester and just cut up the wires. My riocar is currently installed with wires and no harness. I still cant get the illumination to work though.

Check fuses inside and under the hood. In my model black was ground, red was +12, think yellow was always on +12 , I dont remember orange though.