Try to search for DriveSort at Anerty's Lair. I cannot post the URL as a lot of my previous posts were lost when the site was hacked. It works for a lot of car stereos which have the same issue.
Just suggesting to consider making MP3 player from Lumia 650 (weight only 122 g., 16 gb internal memory + SD card slot + 5" great OLED display - one of the best display that I have seen - you can compare it other's flagships displays, like Galaxy S7 Edge's display). It works easily as an offline Navigator too (free world wide offline maps available). Battery will be OK as all connections will be turned off, running in airplane mode with battery saving mode enabled ... you can buy Lumia 650 by 80 euros here, so maybe 60 GBP. btw, it has an FM radio too.