Hi, Stuart
Only the early 90's Interstate offered the aux, in lieu of a tape player. That was a smart thing they offered, for the time. For an aux installation, check out BJ's link:
http://www.sheriart.net/honda/
There's other options, too, like Kennedy, and Sierra. They offer to do the aux install, and/or a kit. I should be doing these retro's, but, there always seems to be someone that will botch the install of radio or switch, and burn the garage down, or some such. So, i just decided to stay away from dealing with the public, as I don't have deep pockets.
I've done quite a few of them, and it's a big improvement. Some players have a weak output, so there's a need for an inline pre-amp, such as a Boosteroo, or something along those lines. I built a "Headbanger" for my 2 bikes, and it works very well. Just Google that. It's on the Mini-disc site. I've thought about building the relay board, like Jon's, along with the "Headbanger" circuit, but have gotten lazy, lately. :roll:
Kennedy has a real nice little board with relay mounted on it. I'd say that's the way to go for a do-it-yourselfer.
http://www.cellset.com/retrofit.html
I hope this helps. Bob