The link below might help. Its written for a 590 and the intention was to get sound out through BT headphones and input via a BT phone, but the settings are made in the same place for output to BT car. Download the pdf in post #1
Problems arise if you have other devices which grab the BT connection first.
At its simplest, you pair the zumo and car. You might have to set the volume, you might have to select the BT type from the icons. But this is all covered in the pdf.
Beware - the Zumo will retain Some separate settings for each cradle. I am not sure if the BT settings fall into this category. The Zumo automatically switches to the settings that were last used in that cradle. Loads of gotchas in there, which I wont go into now.
Anyway, see if this helps. The first time I did it, I had to delete all of my pairings and start from scratch. Then it became simple to set up.
viewtopic.php?f=12&t=520
Have owned Zumo 550, 660 == Now have Zumo XT2, XT, 595, 590, Headache
Use Basecamp (mainly), MyRouteApp (sometimes), Competent with Tread for XT2, Can use Explore for XT - but it offers nothing that I want !
Links:
Zumo 590/5 & BC . . .
Zumo XT & BC . . .
Navigating with Zumo Booklet