FLAC support in car audio equipment is almost non-existent. Pioneer had a model or two that supported it. Sony might have had a model or two as well. I've NEVER seen a factory car head unit that would play FLAC. I've seen WAV support a lot and of course almost all of them support MP3. But never FLAC.
MP3 is probably "good enough" for car use, I agree. But in today's world of storage, I see no need to convert my files if I don't have to. ...and I don't have to with my setup! My setup is a bit unusual; I'll describe it briefly:
I got a 4 year old used Android phone for free from a friend who didn't need it. I installed various customization software to let me remap the buttons and things like that. Then I chose what I thought was the best music player for Android, Rocket Player. I bought Rocket Player (for all of $4) so I could use all of it's features. It plays FLAC no problem, and even plays 24 bit 96kHz audio.
I bought a car mount for it with a jaw that holds the player and a suction cup device to stick it to the dash or windshield. It's cabled to the AUX input on my player and has a charging cable running to a USB cigarette lighter adapter. The charging cable also acts as a "sense" wire and my automation controls when the player automatically plays and pauses based on when it senses that the car is turned on or off.
Because this Android has an SD card slot, I can put quite a lot of FLAC files on a 64 GB card, which is very cheap these days. I don't have to mess with MP3 conversion options; I don't have to wait while the computer does the conversion. I don't have to maintain a cache of (otherwise useless) MP3s on disk. I just copy FLACs to the SD card, insert it into the player and *done*.
My solution is far from perfect. It's had some issues in cold weather. ...and the Android is starting to show it's age. I'm currently shopping for either a ~5" phone to replace it, or for a 7" tablet. I'm not sure yet. I'm almost 100% sure that I can get a perfectly functional device for $100 or under and that's awesome.
I guess this wasn't brief at all.
Good luck,
Brian.