What is an Adapter Harness?
Can I install my head unit without an Adapter?
Why can't I use the adapter I have? (or the one from Crutchfield, K-mart, etc)

OK, so what's an "adapter" and why do I need it?

Simply put, the adapter allows you to connect into the car's factory wiring without damaging or altering it.  The original Honda head unit has a connector on the back that mates with the wiring harness in the car.  That's a good thing, because they must plug together in order to get power from the car to the head unit and audio signals from the head unit out to the speakers.  But that connector does not fit any after-market head unit, so when you buy that nice Alpine or Pioneer radio it will NOT plug into your car's wiring.  Instead, your new radio will come with a short wire harness that has a connector on one end that plugs into the new head unit and open wires (flying leads or pig tails in car stereo lingo) on the other.

In the old days (1970) you would cut the connector off the car's factory harness and splice your head unit wires to it, but that's messy and makes it very hard to put the factory radio back in the car again.  Today, most owners prefer to use an adapter harness that has the special Honda connector on one end and pig tails on the other .  This allows them to splice the head unit wires to the adapter wires, then plug the adapter into the car, leaving the Honda factory wiring and connector intact.  An added benefit to using an adapter is you can do the splicing work outside the car (on a workbench or table) and then just plug the connectors together when you install the new head unit. In addition, using an adapter makes it easy to re-install the factory radio if you sell the car or just want to put the car back to stock. 

Can I install my head unit without an Adapter? - Sure, it just means you'll have to cut the connector off the car's factory harness and splice your head unit wires to it.  This method works fine, just makes it hard to put the car back to stock.

Why can't I use the adapter I have? - Many car stereo shops or stereo manufacturers make adapters to mate with the Honda wiring connector but they're not exactly like the Modifry adapter.  You can use one of these generic adapters to install your head unit but it will not have the Dash Switch and Mute wires.  Without those wires the DCI can't see the dash controls and therefore will not work.  You can of course get around that problem that by cutting and splicing into the factory wiring harness for those two connections, but then you're back to cutting the factory harness again.  Isn't it worth $12 to not cut the factory wiring?