The two low tech approaches are 1) cassette adaptor, 2) FM transmitter.
I've been using the cassette adaptor on my 2000 for years. I've fed satellite radio and MP3 players that way and it worked great. I didn't think it hurt the audio noticably. I use good quality in-the-ear speakers (ear buds) so I am not listening to wind noise that is present with the bike's speakers or even helmet speakers.
I've not used the FM tranmitter method, I've read that there is some sound quality reduction with this approach and it is subject to interference from radio stations, and other radio frequency sources.
I've read here many posts bout Dice and other products that allow swapping out the cassette player for an iPod. I have no experience with this approach, search this site for many posts over the years on the subject. I was deterred from this approach by the cost, potential complexity (I read many posts from folks trying to troubleshoot the setup), and the fact that I didn't mind having the old tech cassette player in the bike.
There were posts about removing the radio, finding, and wiring an input on the back of the radio. If memory serves me, someone had success with the approach, I don't remember what year bike (what bike stock audio system) was used. In any case this approach would be the furtherest from plug 'n play.
My vote remains with the cassette adaptor.