The only way I managed to get sand out of those holes, is to use a shop vac with a cone tip, plug it perfectly flat to the seat, and slap the leather with my other hand in an outward direction (away from the vacuum) while also tapping the seat with the vaccum tip, this seems to get the holes to open up a little and the vibration from the slapping loosens the stuck objects...
Makes you look like you are in a kinky porn film ... but it worked for me.
make sure the gf sees you doing it... for some reason seems to turn them on (if done right) ;-)
again.... it worked for me lol