The adaptor you saw at Crutchfield had RCS inputs, you can plug any device with RCA output plugs into it and play, like a Delphi XM reciever, DVD player, laptop, MP3 player, portable CD player, etc. Also try
http://www.pie.net/apguides.htm for more adaptors. I got one gor my girl's stock Accord radio to plug the Delphi XM radio into without using an FM modulator, which you could also use in your situation. You might also want to check
www.xmfan.com for more info on adaptors. I know you aren't looking for XM, but the same technology applies to the inputs.
Perhaps the easiest and cheapest option would be to use a cassette adaptor that pops into the cassette deck and plugs into the OGG server via 1/8 inch plug.