What's needed ?
Modern cars play mp3's, from CD or USB stick. Old cars don't - however you can purchase a small stereo 'transmitter' that lets you 'broadcast' your own music to the car radio !
Old cars don't have USB sockets, so power will come from a (1A, 5v, USB adapter plugged into) the 12v 'cigarette lighter' socket. To play mp3's, you need to box up a Pi Zero, pHAT-DAC and a standard-sized USB 'socket' (for your USB sitck). If you are lucky (I was) the 'ash trey' next tot he cigarette lighter socket provides the ideal space. An "In-Car FM Transmitter" can be found on eBay for less than £2.50 (from China)
Most FM Transmitters are 'hands free' and controlled from a 'bluetooth' handset = which (like all bluetooth devices) is a real pain to setup, and, if you get it to work at all, then randmoally drops out again = so stick to those that have manual control buttons as well.
I also suggest find one with 3.5mm 'flying lead' (rather than one with a fixed plug designed to plug into your table/phones headphone socket = but watch out = the latest version 'headphone' connector is '4 position' plug and not suitable for use with standard 3.5mm stero socket (such as that on the 'pHAT-DAC').
Transmitters with built-in rechargeable batteries can be powered from the Pi +5v. Others will take 2x 1.5v batteries and can be powered direct from the Pi 3v3 i/o header pins (which can source over 500mA)
Note that it seems most modern versions have a much reduced transmitter power. This means background 'hiss' (no matter what frequency you select) and low sound levels (so you need to turn up the radio volume, which means more hiss and having your ears blasted when the radio auto-tunes to a stronger station). Of course part of the problem is that most older car aerials are mounted on the outside of the car with metal body work between the aerial and the transmitter (more modern cars have aerials incorporated into the windscreen heater conductors).
If you have an actual aerial - especially if it's on the 'roof' of the car - I highly recommend you try out the radio reception with the FM transmitter in various 'pssible' positions within the car before 'building it into' (say) the center console only to find the radio reception is then too poor for it to be usable. Lucky I managed to find an ancient old "Wireless FM Stereo" transmitter with both 3.5mm flying lead and powered by either 2x standard AAA sized batteries (in a 'compartment' that could be relatively easily removed to reduce the size) or an external USB socket (which could be re-wired for use by the Raspberry Pi) One disadvantage of the unit I have is that it has an 'auto-power-off' function. This might be 'good' when running off batteries (which only last a few hours of continuous play) but is a real pain when 'wired in'. Leave a 'gap' of 20 to 30 seconds between tracks (or play classical music with quiet passages) and it just shuts off. To kick it back into action, you have to press the power button = a real pain More modern transmitters have built-in (LiPO) rechargable batteries and/or a USB socket for power. You don't need extra batteries, so these can be removed (and reused elsewhere), but having a USB socket is always an advantage (if it can be made accessible)
Many 'stand-alone' FM transmitters come with a standard USB socket for power input (only). If this can be made 'acccessible' it can be re-wired (to an 'OTG' (micro-USB) cable) for the Pi Zero and used for your USB memory stick music collection !
The typical transmitter has (at least) 2 buttons that can be used to 'tune' it along with a display showing the transmission frequency (so you can manually tune your car radio to that channel). This means the transmitter has to the accessible and visible, so it's the 'last' part to be assembled into the 'ash tray' compartment
Note, make sure to check you get acceptable radio reception before wiring your FM transmitter (and Pi Zero etc) into the ash-tray postion :-)
I note that MP3 'memory stick' players with integrated FM transmitters can now be found on eBay for less than £5. I checked out a number of them on Amazon - there are lots of UK Sellers who buy them from China on eBay at £3 and resell them for £30 on Amazon - so you can usually find the exact same model and read the buyer 'reviews'.
This is where you discover that some allow you to add music but have no way to 'delete' them. Most only support mp3 and WMA (to get WAV support, expect to pay £20+ ). Some include 4 or 8Gb of flash storage and most support plug-in USB stick (often limited to 8Gb), some also support SDHC cards (again, limited size).
The problem with most low cost players is that they tend to deliver very poor audio quality = few have any published 'technical specifications' (such as the mp3 bit-rates supported = you should look for at least 256k (I use 384k)).
The Pi Zero and audio output
The PHAT-DAC can be had for less than £10 abd delivers digital quality sound, so there is no point in using anythng else
The pHAT-DAC has a 3.5mm (3 conductor ring) stereo socket, so a transmitter flying lead' can be plugged straight in
The USB socket
In my case, the 'smallest' part caused me the most trouble :-)
This is where you are going to plug-in your USB stick containing your music collection. So it has to be robustley mounted and accessible from the drivers seat. If the FM transmitter has no USB socket (or does, but you can't find a way to mount the unit so as to make it accessible), then, ideally, you want a 'flat surface' through which the standard socket end of a "OTG micro-USB to standard USB (Soecket) adapter" can 'protrude' (and be gluded into place) If, instead, you have to 'bolt it to the side' of your 'ash-tray' compartment, make sure to use LOTS of tie-wraps (glue alone won't hold it) !
Next page :- KODI fully loaded - (onto the Pi Zero(W))