I still have not got the hang of this after years of trying. I have no difficulty wetting anywhere, any time, while standing or lying down; in public, with friends, it doesn't matter I can just wet when I feel like it without any real thought. Sitting down, I can wet but it doesn't happen freely. A little alarm bell rings because of increased likelihood of leaks and I have to suppress that before I can wet. I am fairly sure this is the more important factor than physiological ones, because I can wet much more easily sitting on a stool in the shower than on the same stool in the bedroom, presumably due to subconscious awareness of the differences in outcome if a leak occurs. At the wheel, for practical purposes I cannot wet the nappy. There have been a few little successes and at one time I thought I was getting to grips with it, but in practice not. No matter how desperate I am, or what posture I assume, it is near impossible to pass any water until the car is parked with the engine off. Then, it's like weeing while seated elsewhere, and IIRC I can relax more easily if I put a waterproof bed pad on the seat.
One day I am going to try putting some crazy absorbent combo on that is 101% leakproof for any amount of flooding and see if that helps release the mental safety interlock. There was certainly a connection between confidence in the nappy and the ease of wetting in public. In the days when I was shy about the nappy being visible under clothes, I used to wear inadequate absorbency and could not let go without preparation because the confidence was not there. As soon as I started wearing thick premium nappies out and about, that had been tested in private to withstand anything I did in them, suddenly the inhibitions melted away. I am hoping the same principle will help with driving but the confidence has to be more extreme because the alarm is more insistent.
None of this applies to messing while driving, which is no different to any other seated scenario and is simply about posture, When the need arises, all I have to do is lift myself up in the seat to make room for the poo, and it happens easily and conveniently.