Is there an adapter that lets you play MP3s (usb drive) in a car that only has cd and cassette players?

There are adapters that do that. They transmit FM to be played over the radio. Many of them are unstable and you have to keep tuning them when they drift off frequency. You have a cassette player though and cassette adapters are cheap. They can't handle USB drives but the can handle ipods and other MP3 players. Cassette adapters work very well.