The simplest is the plain jane RF modulator. Then tune the radio to whatever unused freq the modulator can adjust to.
I always found the cassette adapters better than the RF modulators.
Many aftermarket head units feature auxiliary jacks for this purpose or you can try many model specific adapters... http://www.crutchfield.com/S-2sqkbz0Tqea/m_770/iPod-MP3-Car-Adapters.html
steve said:They have some that will plug directly into the antenna plug, but i have never used one.