Dear Ganeshhariharan ji,
Sai Ram.
You must follow your heart in this matter. If you feel happy and peaceful listening to Shri Sai Satcharitra while driving, you must continue it. But, if you feel an urge to read it also, so you must do that. Since Baba resides in our hearts, He guides each devottee in the way that befits his/her stage of evolution on the spiritual path. If you listen to that small still voice in your inner being, you will get an answer to your dilemma.
Having said that, I think that the emphasis on reading Shri Sai Satcharitra has been laid for the following reasons. In the process of reading, our mind and senses get focused on the text and our attention is fully concentrated on it. During the process of just listening to SSC, we may get distracted by other senses that are involved in other tasks. For instance, while driving your eyes are on the road, your body focused on controlling the vehicle, and your mind divided between these various tasks. In order to avoid distractions, we are advised to read Shri Sai Satcharitra in one sitting. But, that does not mean that someone can not listen to it at other times or read it in breaks. Baba's path is very liberal, so we have no hard and fast rules and regulations in it. Just do what gives you happiness and peace. Hope my humble thoughts help you in some way. May Baba bless you.
Om Sai Ram.