I use a FM modulator and it works great! Couple of notes...
1) Connect it to a toggle switch so you can turn it off. Else you won't be able to listen to your head units FM stations.
2) Ground your case! Else you'll get some really bad engine noise through the lines.
Lastly- Buy the most expensive unit you can! Don't skimp on quality for this piece of your system. I bought a Jensen KVFM1 where you can select one of eight FM frequencies and it also has a level control where you can adjust the realtive level between the FM tuner and the modulator.