On the old forum site, I wrote this in 2017…not sure if it still works
If the car requires a PIN, then it is using legacy pairing from BT <= 2.0. If you are using bluez5 (which you would have actively had to change to on a Jolla 1 i think), then it comes with an autopair plugin, but only tries the code 0000, so if the code is something other than that, then the pairing will fail (i have this on my parrot ck3100).
The solution for me is to disable the autopair plugin. This involves editing /usr/lib/systemd/system/bluethoothd.service and adding the parameter “-P autopair” and rebooting.
If you dont want to edit the file, you can override it by copying it to /etc/systemd/system, and doing the mod there.