burn cd - JC

When I burn music on a cd, I cannot play it on my car cd, but I can on my
portable cd. I am using cd+rw medium. Is there a certain kind of medium I
need to use and how can I tell?

Also is it possible to add music onto a cd that already has music on it? It
seems like you have to erase all data on cd and then add the song to your
playing list and re-record on the cd. Is there a way just to add to the
already existing music?
--
joan