Maybe I'm the only one that listens in this manner, but I tend to listen to CDs where all the tracks are mastered together (an album) whereas with MP3s, I shuffle the songs so I may hear a recording from 1984 followed by a modern one which would likely require some fiddling with the volume knob. Still, even listening to a CD with say, Bohemian Rhapsody, such a box would help in a car with CDs .

I think the only practical way to make an aftermarket device would be to have the processor in the signal chain just before the master volume control device such as a headunit feading an amplifier. If you installed it after the headunit, you would not be able to control the volume effectively because you would just be fighting the AGC in the processor. The only other way would be to cram the processor into the dash or center console somewhere which I don't think is too practical a solution.

Stu