[Kwarde]

pawn Код:

//Above your script:
new LastPlayerInVec[MAX_VEHICLES] = (-1);

//Callback OnPlayerExitVehicle:
public OnPlayerExitVehicle(playerid, vehicleid)
{
    if(GetPlayerState(playerid) == 2) // Is he/she the driver?
        LastPlayerInVec[vehicleid] = playerid;
    return 1;
}

//The function
stock GetLastPlayerInVehicle(vehicleid)
{
    if(LastPlayerInVec[vehicleid] != -1) //Did someone entered it?
        return LastPlayerInVec[vehicleid];
    return 0;
}

Try this ^^

[EDIT]
Damn bmuk was faster