Wow, this gearbox metaphor gained a lot of traction! I'm inclined to see if more torque on the subject would be equally powerful....
So, if we use 'gearbox' as a metaphor for the mind, and we talk about different personality states as 'being in different gears' then we can see that the 'gears' are parts of a gearbox, which is part of a car. (Phew! Everything is neat again!)
But, if 'manual' versus 'automatic' is a true metaphor, then why is Dissociative Identity a Disorder? To answer that, we need to understand how the standard-issue automatic gearbox works. It's really very clever - the gears have got detectors in them, and when the car is trying to do something that the gear isn't very good at, it says "It's time to switch gears now" and a different gear takes over. 5th gear says "Life is a smooth highway" and when life turns out to be something else, we switch to something else. When we're in a parking lot for instance, reverse gear and it's fundamental belief that "There is no way forward from here" finds a way to back up and find a new direction.
DID is a problem with those detectors. So, if we were once pursued by bad guys in black Porsches, then 5th gear might be really reluctant to change down, because it knows that we have to be going really really fast in order to get away from them. And it might refuse to disengage even though we're going up a steep hill, and we'd go faster in a different gear, because it remembers the time it took over by itself when we really needed it, and it saved our life that time.
Meanwhile, reverse gear is looking at the peak-hour traffic, and thinking "There's another car in front of me, I'll find a new direction." But there's a difference (however subtle) between a traffic jam and a parking lot. And there's a car behind us as well, so reverse isn't actually helping! But Reverse remembers that time when a steamroller was coming at us from the front, and it saved our lives that time - it's going to save our lives first, and explain later.
5th and Reverse are not bad gears, they're just not always the best gears. (Sometimes, if we're truly honest, they're the wrong gears, and they look like bad gears because they make trouble in our lives.) And they have good reasons to be scared of each other - if Reverse takes over all of a sudden while we're being tailgated on the freeway, that's a serious problem. And if 5th takes over in the parking lot, that's bad too.
And so, the trick is to find ways to get them to talk to each other, and to learn when these gears are truly useful. Over time, it's possible for them to respect each other and to not be stuck in one gear when another might be worth trying.
Oooh, that's something I've just learned from writing this: If you're really good and comfortable at changing gears (because it takes practice, and those horrible grinding noises and bunny-hops can get quite discouraging), then you can try a gear for a few seconds, and change into another one if it doesn't turn out to be better after all. Hmmmm.