Well there are a few solutions out there.
FM Transmitter (not so hot
IMO)
Tapedeck adapter (much better and CHEEP no static)
AUX in 1/8 inch (Better that tape and cheep too)
AUX in RCA (Most new high end headunits have this.)
Pioneer LINC (Have not seen it in action but it looks nice. I will post more on this when I hear from Pioneer.)
Microsoft SYNC(new Ford option and it looks like it mich be orbecome available as a aftermarket solution. If this turns out to be true.. this is number 1 choice.)
Now this is the new stuff and it's got me excited. It's what I am doing and look for my thread I am making on this. Unless I can get Microsoft Sync in my Expedition
Soundgate/Kicker Solution.