I have a couple of cassette adaptors. And I've seen a cassette shaped mp3 "Mix tape" player that could do basic play/pause/back/forward options.
But does anyone make a cassette adaptor with suitable electronics in it, that allows connection of a usb stick rather than just a headphone connection, so you could plug in any sized usb stick of tunes and play it through the headunit? Like a jumbo version of the Mix tape.
next up option for me is a grom cable that can trick the HU into thinking it has a second changer installed.