I have done this before for my wife's car but I can't for the life of me remember how:

My new car (Audi A3) will read mp3/aac/wma and m3u files off a USB stick. However a lot of my files are now flac, how do I copy playlist and file over whilst re-encoding to mp3 where necessary?

This is the sort of thing that, if I needed to do it, I would have written a custom script to get the job done. Did someone write such a script (either you or someone else), and that's how you did it in the past, and now you need to just remember and find the script?

I think I've just figured it out in winamp. You need to let Winamp manage the device an then you can set sync options

Very nice! I didn't know it could do that.

I hadn't cranked Winamp up for a few years, still works.

Kind of worked, it kept crashing. In the end foobar with one way sync did the job.