Line in with a Belkin cigarette lighter power adapter is a good inexpensive solution. The CD player will take MP3 cds so you can always stick music on them and if you have the navigation radio, you can use compact flash to store music.

The Honda/3rd party solution was pretty lame in that it required you to create special play lists etc.

One 3rd party solution worth investigating is from Dension

We have one these adapters in one of our Audis and it is a very good product.