Mentally "all over the place?" Or, swerving in and out of lanes and traffic?
I'll assume the former, not the latter. The ultimate goal of most practices is to bring mindfulness to everything you do in your life. Mindfulness, as I'm sure you know, is being fully present and focused on the task at hand: if eating, eat; if working; work; if conversing; converse; if driving, drive.
When you sit and practice, you develop the skill of watching your mind without getting caught up in it. When you observe that your mind has wandered, you return to your point of focus (breath, mantra). Observe and return, over and over again. This cultivates the state of mindfulness you can bring to your "non-meditation" time.
Back to driving - you say it's the one area where you struggle. Why is that? What is different between the activity of "driving" vs. other activities where you feel you are mindful? Or, put another way, is there something specific about driving that makes it harder for you to be mindful?
I know you are asking for suggestions, but I feel that exploring the cause of your struggle may help you better deal with it.
Having said that, my biggest suggestion would be a post-it note strategically (and safely) placed where you can see it: on the dashboard, the control panel, the lower corner of your windshield. Just write "mindful" or "where's your mind at?" on it. If it's in your line of sight, you will have a constant trigger to observe where your mind is and return to the present and focus on your driving.
Be safe, and good luck!