[Mauzen]

With the native functions it is not possible.

But you could do something like this: (not tested)

pawn Код:

GetVehiclePassengers(vehicleid, &passengers[4])
{
    for(new i = 0; i < 4; i ++)
        passengers[i] = -1;
    for(new i = 0; i < MAX_PLAYERS; i ++)
        if(GetPlayerVehicleID(i) == vehicleid)
            passengers[GetPlayerVehicleSeat(i)] = i;
}

it should fill an array with the players on the seat 0-3 of vehicleid. If a seat is unoccupied, it returns -1 for the seat id.

EDIT: Paladin was faster, his code should fit perfectly to your needs