Originally Posted by grasshopper
I guess you could first try to make sure that media card support is on within your blackberry. Frankly I don't hold out much hope for this scenario. The way that I have been successful listening in my car is to get a 3.5 mm (male/male) audio jack and plug it into a 3.5 audio port which comes with my car (of course I am unsure if this is an option for your car). I have also been successful plugging an audio adapter into my tape unit of an older car. While researching this question I did find one site that claims to provide a similar feature where you can plug in your device and have it transmit fm signals to your car radio. Note that I have not tried this vendor but it seems to be a viable alternative to your request. Cost is $3.95 - www(dot)softwaresupplygroup(dot)com/arsosfcdmp3i(dot)html
Hope this helps.
Just want to echo what grasshopper said about the 3.5mm male/male cable. Assuming that your car stereo has a 3.5mm auxiliary jack, this approach should work. On my truck, I have been using the male/male cable with good success pretty much the same way I have done with my Ipod. I have streamed music that I synched from Itunes to my Storm; used Moodio fm; and used Slacker radio as well.
As a matter of fact, I liked this approach so well, I did not renew my Sirius satellite radio subscription. Do not miss it at all.