FM Modulator SUCK!!! unless you live in the boonies where theres only 2 radio staions you can pick up. Tape adaptor Best. I used to sell this stuff. No FM Modulators ever works as good as a tape adaptor.
My JVC double din cassette/cd combo head unit also has a front port (jack) for an md player, but virtually any accessory will play through the port. Very convenient!
Several other double din manufacturers offer this feature as well.