The only solution to low volume is to shut out surrounding noise more, which is a bad idea for your use.

There are Line -> Cassette converters, possible retrifits to give you line in on most head units, etc.

It's also generally a bad idea for drivers to wear earphones - the situation is a little different to listening to a car stereo since earphone listening takes attention slightly more away from the surrounding environment in comparison to reasonable-volume listening with in-car entertainment.