The software that runs the head unit in the car probably isn't compatible with iTunes Match. We've seen this reported before with other cars. Try downloading some tracks and see if they play through the sound system. If they do you'll want to contact the manufacture of the head unit to see what they have to say.
Very common problem. Most built-in i(whatever) interfaces are pretty bad and don't keep up with new Apple features.
In my case (Nissan Cube), it would "see" the Match content (that is, list it), but not play it unless *every song* in the playlist or album was downloaded (either by playing it first, or clicking on the cloud icon). Maybe this will work for you, too.
Add to that the extremely limited interface, and in the end I simply gave up and started using the AUX plug. I've been happy ever since.