It's possible you wet yourself during the serious initial crash and so your body is going back to that. I would start preparing yourself for rides. Make sure you empty your bladder before you get in the car. Try to avoid being a passenger whenever possible. Maybe bring a towel to sit on. PTSD is frustrating because we have these physiological responses that happen automatically and we have no control over them. Part of it is learning to be aware of how you are feeling, when you are getting anxious or tense, talking yourself down etc. Part of it is setting up situations so you aren't triggering yourself, and part of it is just preparing the best you can.