Definitely Not About Piss *cough*

You kinda have a remote detonator. With a push of a button, they pee themselves.

Also the idea of having a vibrator in me. And my partner is waiting for me to least expect it.

-Trying to squirm subtly because they’re right next to the driver, bouncing slightly in the seat, adjusting position, shaking their legs.

-Someone trying really hard not to grab their crotch because it’s embarrasing and makes it obvious they need to go, but at a bad spasm they’re forced to. The driver notices and asks them if they need to go to which they either shyly admit feeling silly they had to be asked like a child or shake their head and look even more like a child denying they need to go when they clearly do.

-Someone waiting until they’ve already dribbled to ask for a rest stop. The driver says sure there’s one in fifteen minutes, but they’ve waited so long already they wet themselves after ten. 

-Alternatively, never asking for a rest stop and slowly leaking the whole journey. They never fully lose control but by the time they’re at their destination they’ve leaked enough that there’s a wet patch on their pants and seat, and they don’t want to get up because then the driver and other passengers will know what’s happened.

-The pee making a puddle on a leather seat and running over the edge or soaking into a cloth seat making a big wet patch

-Someone making it to the rest stop and running in only to lose it before they can undo their belt and being forced to walk back to the car in soaked pants. 

-pulling over at a rest stop that has bathrooms, but they’re so desperate they barely make it out of the car and have to settle for the nearest bush / patch of grass before the floodgates open, how sympathetic the other characters are depends

-being pulled over by the cops for speeding when they were only doing so to get to a bathroom faster, now, of course, they have to wait even longer while getting lectured

-trying to pee in a container but something goes wrong, it overflows, the driver hits a bump in the road and they mess up their aim, the container wasn’t water-tight..

But you don't stop, oh no, hearing the broken moans only spur you on, pounding into them harder and harder until you hear that final embarrassed squeak leave their lips as the first spurt comes out

They try so hard to stop, their muscles tightening and fluttering as much as they can but it's not enough when you're hammering into them mercilessly, small spurts turning into a stream, and a stream turning to a gush as they moan and finally relieve all that pent up pressure

They're so lost in the relief that they don't realize how close they are, their hips bucking back onto you further until suddenly they're cumming around you, the climax hitting them like a ton of bricks. You follow suit soon after, stilling your hips and burying yourself deep inside them, filling them up nice and full