I had a similar problem both the FP5 and now with the FP6, it’s described here: Is there a way to check the USB-C port health status?
What does it happen if you connect your phone to your car through Android Auto? Does it work?
Something that helped me (at the beginning then things got worse) was connecting and disconnecting the cable, waiting every time a couple of minute to see if the charge increased. If it increase don’t touch it for a while and keep your fingers crossed, it could charge.
Remove the “eco charging” feature if you’re using it so that you can take profit of any minute the charging works. Remove also the limit to 80% charge so that you can fully recharge the battery. Finally, use the “battery saver” function to save power.
Another thing you could try is charging it when it’s turned off, this was also useful a couple of times but at the end, as I wrote, things became unmanageable and I solved the problem only by replacing thee USB-C port module.
Don’t wait, open a ticket to the support and ask for help: https://support.fairphone.com/hc/en-us/requests/new
Ciao,
Max