I had been using Replicant on my S2 for a while. It crashed a lot and failed to install many apps, so, finally I replaced with my own built Sailfish Os.

Regarding you car multimedia:
Yeah, probably the routing is not correct. Unfortunately don't use car multimedia with my phones, so cannot really test what the correct routing should be.
What I can advise you, install a Sailfish Os app called Audio Output Chooser from Chum and try whether you can route the sound properly to your car multimedia. Then, post your test results here if that helps others.