Any hack for an old car radio?

[CelticRambler]

The camper has a good, reliable old-fashioned RDS AM/FM radio with optional multi-deck CD input (unused) but no other auxillary inputs. On my long, long, long commutes, I like to play movies (as MP4 files) through the radio, using one of those little FM re-broadcasting thingies that you plug into any device's 3.5mm audio-out socket.

But these days, the FM frequencies in France are intensely cluttered up with local radio, and it's become almost impossible to find any frequency to use for more than about 50km's worth of a drive (and sometimes not even that).

Other than replacing the radio, is there any other way to feed the audio output from e.g. a smartphone or tablet into the unit? It's a Panasonic CQ-RDP003N