new LastAttemptedJack[MAX_PLAYERS] = {INVALID_PLAYER_ID, ...};
new LastEnteredVehicle[MAX_PLAYERS] = {INVALID_VEHICLE_ID, ...};

stock GetVehicleDriverID(vehicleid)
{
    for(new i = 0; i < MAX_PLAYERS; i++)
    {
        if(IsPlayerConnected(i))
        {
            if(GetPlayerVehicleID(i) == vehicleid && GetPlayerState(i) == PLAYER_STATE_DRIVER)
            {
                return i;
            }
        }
    }
    return INVALID_PLAYER_ID;
}

public OnPlayerEnterVehicle(playerid, vehicleid, ispassenger)
{
    if(!ispassenger)
    {
        LastAttemptedJack[playerid] = GetVehicleDriverID(vehicleid);
    }
    return 1;
}

public OnPlayerStateChange(playerid, newstate, oldstate)
{
    if(newstate == PLAYER_STATE_DRIVER)
    {
        LastEnteredVehicle[playerid] = GetPlayerVehicleID(playerid);
    }
    if(oldstate == PLAYER_STATE_DRIVER && newstate == PLAYER_STATE_ONFOOT)
    {
        LastEnteredVehicle[playerid] = INVALID_VEHICLE_ID;
    }
    return 1;
}

public OnPlayerDeath(playerid, killerid, reason)
{
    if(killerid == INVALID_PLAYER_ID)
    {
        new jacker = GetVehicleDriverID(LastEnteredVehicle[playerid]);
        if(jacker != INVALID_PLAYER_ID)
        {
            if(LastAttemptedJack[jacker] == playerid)
            {
                killerid = jacker;
            }
        }
    }
    return 1;
}