Years ago I bought one of these cigarette lighter FM transmitters for the car - you can play music from a USB stick over FM and pick it up on the radio.
As we're Oop Nowerth this weekend - West Kirby to see big bro', thence to the North Yorkshire Dales via Accrington to see No.1 daughter - I thought I'd dig it out and see if it still worked, as the CD changer is the Merc is not connected (damned thing has a 10 pin DIN, whereas the radio has an 8 pin DIN). It was swamped by local stations within the narrow band in which it transmits, so next to useless.
When these gizmos first came out there weren't so many FM stations, but since then they have proliferated like inane and unenlightened Facebook posts. Not worth buying, unless you do a lot of driving in the sticks.
Now an app for the phone that transmits on selectable FM (assuming the makers could put an FM transmitter into the phone) would be a brilliant idea. Use Waze for routing and an app to play my music over the radio.