small audio compressor/ limiter for car?

For years, I've been using a tiny FIIO amp ahead of my car MP3 player to
play my mostly classical music. The amp has a cassette adapter after it
that allows me to play the system through my car's stereo. While this
has worked pretty well and, to my surprise, the tiny FIIO amp has lasted
over nine years, I've always wanted to add a small compressor/ limiter
between the MP3 player and amp. Oh, I've done some of the compression
in software, but for many, many MP3's, this takes a lot of time and have
been wondering for quite some time if there was a small unit about the
size of the FIIO that is of decent quality and not too expensive that
would be suitable? Thanks.